MorseKOB Interfaces

From an early age I’ve had an interest in historical electrical communications systems such as the electrical telegraph, local battery telephone systems, Morse code over radio and mechanical teleprinters.

That’s why I was delighted to find Les Kerr’s MorseKOB system ( ) while surfing the internet.  Les’ MorseKOB is a system that allows an individual to connect (through a simple electronic interface) a telegraph key and sounder to a Windows computer, and then connect the Windows computer to a multi-circuit telegraph conference bridge via the internet.  Some of the “wires” (channels) of the conference bridge can be used to communicate via telegraph instruments with other individuals anywhere in the world who have internet service, either one-on-one or in party line fashion.  Several other “wires” are broadcast channels that continuously send a variety of content, such as current news stories, historical news stories, weather information, and even OS (reported On-the-Sheet) train movement reports.

The Windows MorseKOB Ver 2.5 is a pretty amazing program. However, it is a legacy program having been written in 2009, and as Windows has evolved there are now a few quirks. I’ve added some notes to the bottom of this page on how to work around those quirks.

I’m moderately proficient with international Morse code, but have little ability with American Morse.  I hope that, when time permits, that I can gain some conversational ability with American Morse.  But until that happens, I enjoy having a MorseKOB enabled sounder make telegraphic music when I’m in my workshop.

With that as background, I’d like to describe my own implementation of a MorseKOB interface.  I’ve done three different versions of a MorseKOB interface, with the most recent one including an on-board USB-Serial interface and using only very common TO-92 package BJT transistors to drive the sounder. I think a MOSFET may provide better switching for the sounder loop than a BJT, but many contemporary MOSFETs are available only in surface mount and are subject to the world-wide-chip-silicon shortage. The TO-92 high voltage BJT transistors are still readily available and cost about $3 for a lot of 100 pieces.

All of the interface designs that follow are very similar to Les’ original design; I just used a few different components.


Figures 1 and 2 below show the USB-Serial version of the MorseKOB (the USBmorseKOB). This device was designed to be used with 4 ohm local sounders, but with a few component adjustments it can be used with sounders of up to 400 ohms. The values of R1 and R12 are selected in consideration of the the driving voltage at J2 and the normal operating current of the sounder. The values on the schematic assume a 4 ohm sounder with a driving voltage of 5 volts and a normal sounder magnet current of ~250 mA. I use a 5V-1A wall wart to provide loop current for the USBmorseKOB. Power for all the transistor circuits on the USB MorseKOB (but not the sounder current loop) is provided via the USB connection. In order to achieve “snappy” sounder operation, the L/R ratio of the loop circuit must be relatively low. The good news is that a 4 ohm sounder doesn’t have much inductance, about 5 mH, so that a relatively low current limiting resistor and relatively low driving source voltage can be used. The USB connection to a PC comes into either J1 (standard size USB connector) or J4 (mini size USB connector). Use only one of these USB jacks at a time. U1 is a CH340C USB-Serial bridge chip that requires no crystal. It breaks out the necessary DSR* and RTS* signals from the USB link (“*” appended to a signal name means that that signal is active low). I get the CH340C devices from for about 60 cents each in lots of 10. They are in an SOIC package with pins on 50 mil (1.27mm) centers, but I find them pretty easy to solder even with my old eyes and less than surgeon-steady hands. I’ve found these devices to be virtually foolproof, no issues.

The signal to the sounder is presented by U1’s output RTS* lead . The RTS* signal is inverted and buffered by Q2, Q3, and Q4, with Q4 sourcing about 4.9VDC on its emitter when the RTS* lead is low. D2 ensures that there is no back-feed current when a PC is not connected to the USB jack. PH5 is normally open – more on that later. With Q4 emitter at +4.9 volts, the voltage across R11 is about 4.1VDC and LED DS1 is illuminated to show that the magnets should be in the energized (marking) state. Additionally, when Q4’s emitter is at +4.1VDC transistors Q5, Q6, Q7 and Q8 are all “on” with a healthy 3+ mA of base current into each transistor. The set of four transistors can be driven in parallel because of the 3.9 ohm ballast resistors in the emitter leg of each transistor will help to balance the collector currents between the transistors. Typical values for Q5-Q8 base voltage is 1.0 VDC and emitter voltage is 0.3VDC. The current into the collector of each transistor (to test for balance) can calculated by measuring the voltage across each 3.9 ohm resistor, then dividing by 3.9 (ohms law I = V/R), and then subtracting about 3mA for base current. Q5, Q6 and Q7 are MPSA42 transistors which can tolerate a voltage from collector to emitter of up to 300 VDC. I get these on ebay and AliExpress for about $3 per hundred.

When RTS* is high, then Q4 is in cutoff and the voltage across R11 is zero and Q5-Q8 are cut off and no current flows through the sounder, regardless of the key open/closed position. When Q5-Q8 are on and the key is closed loop current of about 270 mA flows through the sounder. In this state the voltage at the emitter (3.9 ohm resistor) of Q5-Q8 is about 275 mVDC, the voltage at the collectors of Q5-Q8 (and thus cathode of D3) is about 790 mV, and the voltage at the anode of D3 is about 1.6 VDC. The voltage threshold at the anode of D3 to turn on Q1 is about 1.0 volts. When 270 mA of current is flowing through the sounder, D3’s anode voltage is about 1.6V and Q1 is ON. When current does not flow through the sounder due to an open key, then the voltage at D3’s anode is very near zero and Q1 is off. The state of Q1 is passed on to U1 via the DRS* lead. DSR* is low when the key is closed and high when the key is open. D4 is a clamping diode (think of it as a hefty zener diode) that will limit the positive voltage peaks to about 150 V (will likely change this device to an 18V device in the future) and the negative voltage peaks to about -1 volt.

PH5 is a shorting block that is normally open. If you wanted to use this interface without a computer, to just have a simple loop circuit so that the key would operate the sounder in local mode, then ensure that the USB cable is disconnected from the board and install a shorting jumper on PH5. That will provide base current to Q5-Q8 and the key will now operate the sounder without any computer control (and without any +5 power provided by the computer). The value of R12 must be sized to the voltage source used for the sounder loop. In general, choose a value that results about 4 volts at the cathode of D2 when PH5 is shorted.

What follow are some comments about using a loop current of 40 mA (120 ohm main line sounder). In this case the driving voltage is increased from 5VDC to 12VDC and the total loop current limiting resistance is changed to 120 ohms. An easy way to accomplish that is to add an external 120 ohm resistor in the loop circuit (the extra 6.8 ohms on the board won’t really matter in this case). With these changes in place and 40 mA of loop current flowing, the voltage at the emitter (3.9 ohm resistor) of Q5-Q8 is about 55 mVDC, the voltage at the collectors of Q5-Q8 (and thus cathode of D3) is about 85 mV, and the voltage at the anode of D3 is about 0.8 VDC. Note that this D3 Anode voltage is not high enough to turn on Q1. An easy fix for this problem is to make a change at D3. Instead of using one diode at D3, for loop currents of less than 100 mA use two diodes in series for D3. Additionally, with the larger driving voltage of 12 VDC used with 120+ ohm sounders, the value of R12 should be changed to 470 ohms. The goal being that when nothing is connected to the USB jacks and PH5 in shorted, the value of R12 should be selected such that the voltage at the cathode of D2 should be about 4 volts.

The photo below shows the assembled board, which is 2″ x 3.9″. The USB connection is on the left side of the board. The power input is on a coax power jack (2.1mm ID, 5.1mm OD) at the upper right side of the board and the key-sounder loop is on the Tip and Ring contacts of the 3.5mm TRS jack on the lower right side of the board.

Click on the photo above to see it in higher resolution

Below is the schematic of the USBmorseKOB board. Click on the schematic image below to open the schematic as a PDF in a new window.

Figure 1 – Schematic of USBmorseKOB board (Rev 3)

Click HERE to see the above schematic as a PDF

Click HERE to see a PDF of the Assembly Instructions (Rev 1.1) for USBmorseKOB board (Rev 3)


*PARTS: MPSA42s are very common inexpensive transistors that can tolerate collector-emitter voltages of up to 300 V. But they can’t pass a lot of current, so to handle 300 mA of sounder current several MPSA42s transistors are needed in a parallel with emitter ballast resistors to equalize the currents. For my older designs that used MOSFETS, good choices for the MOSFET are IRL620, IRL640, IRF620, IRF640. The IRL parts have a gate threshold voltage of about 2.5 volts while the IRF parts have a gate threshold voltage of about 4.5 volts. If you are buying parts rather than pulling from your junk box, I’d suggest you get the IRL640 as that would be the most flexible (lower gate threshold voltage, higher drain source voltage). In many applications LED current is set for about 10 mA for good visibility. In this case, I want a dim indicator, not a lighthouse. So, I used a 10K ohm resistor and my LED is a high efficiency type. Adjust the value of the LED current limiting resistor to give you the brightness (or dimness) you want in the visual indicator. I have lots of parts, because I often buy in lots of 10 or 25. If you need parts feel free to contact me to see what I can offer for free. If I have spares I’ll be happy to give you some so you can make your own interface. I may ask you to cover postage.

GROUND: Earth ground is your friend. There’s a basic rule in electrical design that says “no floating metal”. All metal masses in the system need to be committed to a known voltage relative to earth ground. The known voltage can be time varying or AC, but it must have a galvanic connection to earth ground (even if it’s through a 1 Meg ohm resistor). In order to be a safety ground, rather than just a static dissipating ground, the path to ground must be one of low resistance. Like most telegraph systems of old, the exposed metal of the telegraph key is not at earth ground potential. In most cases the voltage on the key is relatively low, but if you are using a loop supply with an open circuit voltage of more than 48 volts you need to keep in mind that the key is “hot”. When your computer is connected to the AC mains via a 3 wire power cord, the circuit ground on the USB cable will likely be at earth ground potential, and that provides the earth ground reference for your MorseKOB system. If you are in doubt about earth grounding via the USB cable, disconnect your computer from AC power and check with an ohm meter for continuity between the MorseKOB ground and the ground pin on the computer power cord. When using a laptop computer from internal battery, without a line cord from laptop to AC mains, it’s likely that your MorseKOB circuit common is not at earth ground potential. In this case it would be good practice to have a ground wire from your MorseKOB circuit common to some source of earth ground. A ground wire from the MorseKOB circuit common that ends in a banana plug (with the contact vanes bent outward a bit) that is inserted into the ground pin of an AC outlet will solve that problem. One could also use a connection to a cold water pipe (assuming it’s metal pipe all the way to where it hits dirt) or even a ground rod. All this may sound excessive, but it’s really just good practice.

FUSES: Do you really need a fuse in your loop supply? I think the answer is yes. Fuses are cheap and they provide protection. Telegraph sounders are not easily replaced in the 21st century yet fuses are readily available. So just do it. Use a fuse. I don’t expect the fuse in the MorseKOB interface to ever need replacing, unless I do something stupid, which does happen. When I hand build breadboard prototypes I usually don’t use a fuse holder. Instead I just solder the fuse into the circuit. To make that easier, I cut off the leads of a 1/4 watt resistor and then reuse the leads by carefully and quickly soldering them to the ends of the fuse body. Parts catalogs show leaded low voltage fuses, but the minimum order quantity is often 1000 pieces. Minimum order for low voltage fuses intended for a fuse holder is 1, so I buy those and add my own leads to the fuses that I use in breadboards.

LOOP SUPPLY: The loop supply for a MorseKOB interface should be configured as shown in the schematic: A series circuit composed of a DC voltage source with negative side grounded or connected to circuit common, the positive side to a fuse, then the other side of the fuse to the loop current limiting resistor, and then the other side of the resistor is the “output” of the loop supply. The output of the loop supply would then be connected as a series circuit through the sounder and key, to the MorseKOB interface which returns current back to ground or circuit common. The fuse should be rated for about 1.5 to 2.0 times the expected typical loop current. If you connect the output of the loop supply (composed to voltage source, fuse and resistor) to ground for 30 minutes the fuse should not blow and your loop current limiting resistor should not fail. If your loop current limiting resistor gets too hot touch (but is still within the resistor wattage spec) you may need to provide for some ventilation holes in your loop supply enclosure.

L/R TIME CONSTANT: The inductance of your sounder magnet coils goes up as the resistance of the coils in your sounder goes up (longer wire, higher resistance, more turns yields more inductance). As the inductance goes up, you will likely need larger value of current limiting resistor and a higher voltage loop supply, in order to have the sounder produce “snappy” action. When the sounder magnet inductance gets higher (think 120 and 400 ohm sounders) the only reasonable way to lower the L/R time constant is to increase the series resistance. Then, with a larger loop resistor in circuit, to proper loop current so that the sounder armature with strike the anvil with some gusto, the voltage source needs to increase. A good rule of thumb to achieve “snappy” sounder action may be to use a current limiting resistor that is 1 to 3 times the value of the sounder’s internal resistance and then choose a driving voltage that results in the desired current flow. For a 4 ohm sounder I use a current limiting resistor of 6.8 ohms and a driving voltage of 5 volts. For my 150 ohm sounder I use a current limiting resistor of 120 ohms and a driving voltage of 12 VDC. As for sounder inductance, for my 4 ohm sounder I measured about 5 mH of inductance and for my 150 ohm sounder I measured about 500 mH of inductance.

DIODE VOLTAGE REFERENCE: Many electronics hobbyists (sometimes myself included) fall into the trap of thinking that the voltage drop across a silicon diode, or base emitter junction of a transistor, is always exactly 0.7 volts.  It’s not; it’s approximately 0.7 volts, but the actual value will vary depending on the amount of current passing through the diode junction and the temperature of the diode.  According to its data sheet, a common 1N4148 diode at 25C (77F) will have a voltage drop of about 0.7 volts at 5 mA;  0.5 volts at 100 uA;  and 1.0 volts at about 180 mA.  Temperature is an additional factor: higher temperature leads to lower voltage drop, and vice versa.  It’s OK to use 0.7 volts as an approximate value for voltage drop across a silicon diode, but always keep in mind that it is an approximate value.

SNUBBERS: The electrical component of a telegraph sounder is an electro-magnet, which can have a significant value of inductance.  As the old adage goes “the current through an inductor can not change instantaneously”.  So, when the key of a sounder loop goes open, the supply of current to the sounder is cut off, but then consider that “the current through an inductor can not change instantaneously”.  What that means is that when the key goes open (or the MorseKOB transistor goes open), the inductor, at least for a few microseconds or milliseconds, as the magnetic flux collapses, will try to force that same amount of current into that open circuit.  So if there was 100 mA sounder current flowing, and then the key goes open, the sounder “sees” an infinite resistance (because the circuit is now open), and tries to shove 100mA of current into that infinite resistance (for a few microseconds or milliseconds).  The resistance of the open circuit is not actually infinite, as there are always leakage paths, but the open circuit resistance IS BIG.  Let’s assume it’s 10,000 ohms (although it’s probably much larger).  Ohms law applies, so the sounder wants to send 100mA into 100,000 ohms, at least for a few microseconds.  So 0.1 amperes x 10,000 ohms equals is 1,000V.  That voltage spike creates two problems. One is that the high voltage spike can damage loop switching transistors on the MorseKOB board and the other is that these fast voltage spikes will produce radio frequency interference (RFI). The RFI may not be of much concern to you, unless you are operating near an HF radio receiver, but the voltage spike can damage the loop switching transistor in the interface circuit. Which brings us to “snubbers”. Snubbers are circuit elements that limit the amplitude of the voltage spike produced by an inductor when the current flowing through the inductor is interrupted quickly. Western Union apparently used a 22 ohm resistor in series with a 100nF (0.1 microfarad) capacitor as a snubber across the sounder (and perhaps relay) coils. I use a series RC network as well, but I also add an additional 1nF (0.001 microfarad) capacitor in parallel with the coils for my snubbers. Without the 1nF capacitor, when the key (or Q1) goes open, the voltage spike will be limited in amplitude by the resistor in the snubber, but the voltage will jump up to that limited amplitude very very fast, almost a step function, which will produce a click on a radio receiver. The 1nF capacitor is effective for only a few microseconds after the key (or Q1) goes open, but it will slow the rise of the voltage that appears across the snubber resistor and will greatly reduce the key clicks as heard on a radio receiver. So, to summarize, the 1nF snubber cap is to kill key clicks as heard on a radio receiver and the the 100nF cap in series with a resistor are to limit the peak voltage produced by the sounder coil when current through the sounder coils is interrupted – I used both at the same time. My snubber resistor of 470 ohms was picked because it’s what I use for Teletype magnets. That may not be the best value for telegraph sounders. I need to do more testing to see what the best value may be. Also, the best snubber resistor value for a 4 ohm sounder may not be the best value for a 120 ohm sounder; but something is probably better than nothing. It’s best to mount snubbers as close to the inductor (sounder magnets) as possible. I mount my snubbers on the bottom of the sounder, where they are less likely to be visible. Keep in mind that semiconductor components don’t always die instantly from a single large voltage spike.  In some cases it can take hundreds or thousands of spikes before ill effect is evident. However dead components are dead components, regardless whether they die with first voltage spike or the 10,000th voltage spike.

Having said all that, D4 on the USBmorseKOB board is like a 150V zener diode. It’s purpose is simply to protect the MPAS42 transistors from excessive voltage from collector to emitter. Some telegraph enthusiasts have suggested using bilateral protection diodes (like two zener diodes in series) across the sounder magnets, and I think that adding those is a great idea. Clamp voltages of about 7 volts are recommended for 4 ohm sounders and clamp voltages of about 18 volts for 120 ohm sounders. Place them right at the sounders terminals, under the wood base, so they are not readily visible.

MorseKOB 2.5 Windows 10 Quirks and Workarounds

  1. MorseKOB 2.5 appears to only work with serial COM ports from 1 through 9.  It does not appear to work with COM ports of 10 or higher.
  2. When you connect a USB to serial converter to a PC Windows 10 will assign that converter to a particular serial COM port with a numeric value of somewhere between 1 and 256 (COM3, COM21, COM103, etc).  As a matter of convenience, once Windows 10 assigns a particular COM port number to a particular USB device, it will reserve that port number for future use by that device, even when it’s not connected.  So, if you have connected a lot of serial devices to your computer over its life (or since the last re-install of Windows), it’s possible that your new USB to serial converter will be assigned to a port number in the 10s or 20s.
  3. When you connect a USB to Serial converter to your windows computer, the only way that I know what COM port number has been assigned to it is to find it in the Windows “Device Manager”.  To find the COM port number of your USB to Serial converter, right-click on the Windows icon on the lower left part of the computer display.  Then left-click on Device Manager from the pop-up list that appears on the left side of the screen.  Look for “Ports (COM and LPT)” and left-click on the very dim and hard to see right-pointing-arrow-bracket just to the left of “Ports (COM and LPT)”.  And under “Ports (COM and LPT)” you should see a list of connected serial devices.  If there’s only one in the list, then it’s obviously the one you have connected.  If you have two or more in the list, then you may need to disconnect and reconnect your new USB to Serial converter to determine which COM number has been assigned to that device (you will see it appear in the list when you connect it to your computer’s USB jack, and then disappear when you disconnect it).
  4. If your USB to Serial converter is assigned a COM port with a numerical value of 1 through 9 inclusive your life just got a little easier.  Note the COM number and go to step 7 below.  If you have a COM port number of 10 or greater, you will need to “reassign” the COM port to a number 9 or lower.  Note your current COM port number for your USB to Serial converter and go to step 5 below.
  5. To reassign a COM port to a different number, here’s a procedure.  First, in Device Manager note which COM port values between 1 and 9 are *NOT* currently in use.  Pick one of the NOT IN USE values between 1 and 9 inclusive, and make a note of it.  I try to avoid numbers 1 through 4 because some of my computers have internal serial cards that seem to be restricted to COM Ports 1 through 4.  Let’s assume that COM6 is not in use at the moment on your computer, and that we are going to reassign your USB to Serial converter to COM6.
  6. Under device manager, find your USB to Serial converter based on the connect/disconnect method in step 3.   Right-click on the COM port in Device Manager and then left-click on Properties.  Then left-click on the Port Settings tab and then left-click on the Advanced button.  In the lower left part of the new screen is an area where you can pick a new COM port number.  Left click the pull-down button and select COM6.  Don’t worry about the “(In Use)” notation if it appears, we determined in step 5 that COM 6  was not in use at the moment.  Left-click OK.  Don’t worry if you get a warning such as  “This COM name is being used by another device… yada yada yada.  Do you want to continue?”.  Left-click OK if you get that warning.  And then at the next screen left-click OK.  And if needed left-click OK at the next screen.  As of now Device Manager should show your new USB to Serial converter assigned to COM6.  Whatever device Windows had previously reserved for COM6 will be automatically assigned a new unreserved COM number when it is reconnected to your computer.
  7. To ensure that maximum likelihood of success, now would be a good time to RESTART your computer.
  8. Open MorseKOB 2.5 (and no other applications on your PC at this point).  If you get an “Unable to open serial port…” message then left-click OK.   Left-click Tools, then left-click Preferences.  In the right side of the new window, under Configuration, left-click on the Port pull-down and select NONE and left-click OK.  Then left-click File and then left-click Exit to close MorseKOB 2.5.
  9. Open MorseKOB 2.5 again.  Because in step 7 above you set the Port to NONE, you should not get a port error message at start up this time, since no port has been selected.  Next, left-click Tools, then left-click Preferences.  In the right side of the new window, under Configuration, left- click on the Port pull-down and select COM6 (or whatever port your USB to Serial converter is assigned, but it must be 9 or less) and left-click OK. From the main MorseKOB pick a wire number (I like 109) and left-click Connect.  That should get you going.
  10. If you should ever get the “Unable to open serial port…” message in the future, check device manager to confirm that your USB to Serial converter is still at your desired COM port assignment (COM6 in the case above) and then repeat steps 7 and 8 above.



I’ll first discuss the RS-232 version. Figure 2 (further down the page) shows the schematic of my version of an RS-232 to MorseKOB interface.  You will see that it’s very similar to Les’ original design.  Some of the changes I made were to use an LM358 as the op-amp and a MOSFET as the switch.  I also added a dim LED to serve as an RX Mark state indicator.  Additionally, I’ve included a voltage spike snubber across the sounder coil.  This is essentially the same snubber that I use on selector magnets in teletype machines.  I need to do more testing with these snubber values to see if further optimization can be achieved for the different sounder windings and their associated inductance.  With that as background, let’s get into the RS-232 interface circuit.

One key component not shown on the schematic is a USB to RS-232 serial converter itself.  This is a device that connects to a USB port on a computer and presents an RS-232 terminal interface to a modem (the modem in this case being the MorseKOB Interface).  These devices usually look like a cable with a USB-A connector on one end and a DB-9 male connector on the other.  These devices are available from many vendors and cost between $7 and $15 dollars.  The DB-9 male connector on the converter cable connects to P1, shown at location D1 on the schematic.

In conventional serial port operation, the serial data from the computer to the modem would appear on pin 3, the TXD lead.  For the MorseKOB system the serial data from the computer to the sounder appear on pin 7, the RTS pin.  This RTS signal is bipolar, swinging from approximately -6 volts for sounder off to +6 volts for sounder on[1].  The RTS signal is passed to the gate of Q1, the MOSFET switch, through resistor R9.  Because of the extremely high impedance of the Gate pin to Source pin on Q1, there is no substantive voltage drop across R9.  R8 ensures that the gate is grounded if the RTS signal should ever go into a high impedance condition.  An “RX Mark state indicator” is formed by LED DS1, R7 and D4.  DS1 is a high efficiency LED so that the current needed for the LED is low (less than 1mA) and I want the LED to be dim, not bright.  D4 is included because some LEDs will tolerate only a small amount of reverse voltage.  Although Q1 physically appears to be a “beefy” device and is constructed to easily mount on a heat sink, in the MorseKOB application Q1 acts as an on-off switch, dissipating virtually no power, so a heat sink is not needed or desired.

When the gate of Q1 is more positive than about +2,5* volts, Q1 will turn on, conducting current from Drain to Source[2] when the telegraph key is closed, thus causing the sounder to pull its armature down onto its anvil.  When the gate of Q1 is less than +1 volt (keep in mind that “less than +1 volt” includes negative voltages) Q1 is in the off state and no current flows from Drain to Source, thus interrupting current flow through the sounder.  Note that virtually all of the current that flows through the key and sounder also flows through Q1 and out the Drain pin of Q1, passing through R10 (1 ohm) to ground.  R10 has no operational effect on the circuit, other than it provides a means to easily measure the current that is passing through the sounder.  For every millivolt across R1 there is a millampere of current passing through the sounder/key loop.  So, if the voltage across R1 is 150 mV then the current through the sounder is 150mA

The purpose of op-amp U1A and associated components is to inform the computer, via the DSR signal, the state of the telegraph key, without regard to whether Q1 is conducting current or not.  Let’s look more closely at how this works.

An interesting aspect of the RS-232 MorseKOB interface is that power for the op-amp is derived from RS-232 signals themselves, in particular the DTR signal for a positive voltage source and the TXD signal for a negative voltage source..  When running the MorseKOB software both the TXD and DTR signal leads stay at there respective voltages and do not change.  A reference voltage of approximately +0.7 volts for the negative op-amp input (pin 2) is developed by R3 and D1.  The positive input op-amp input (pin 3) monitors the voltage level at the Drain of Q1, raised by approximately 1.4 volts due to the voltage drop across D5 and D6 when current flows through them.

When the key is open, regardless of the state of Q1, the voltage across R5 is zero (ignoring op-amp input bias currents which are 10s of nano-amps) and thus the voltage applied to the positive input of the op-amp is zero.  In this condition the output of the op-amp (DSR lead) goes to (near) the negative supply voltage, -6 VDC, telling the computer that the key is open.

When the key is closed and Q1 is off (no sounder current flowing), the voltage across R5 is nearly equal to the loop supply voltage, somewhere between +5 and +130 volts.  The voltage across R5 is also applied to the clamping circuit formed by R6, D2 and D3, to keep the voltage applied to the positive op-amp input between DTR+0.7 volts on the high side and -0.7 volts on the low side.  Additionally, the current flowing into the op-amp due to excessive voltages is limited by R6.  Regardless of the specific voltage of the loop supply, when the key is closed and Q1 is off, the voltage on the positive op-amp input is greater than the voltage on the negative op-amp input (+0.7 VDC) and in this condition the output of the op-amp (signal lead DSR) is near the positive supply voltage, +6VDC, telling the computer that the key is closed.

Now consider the case of key closed and Q1 fully on, conducting current.  Also imagine that D5 and D6 were shunted by a shorting strap, so that they were no longer operational.  In this case with the key closed and Q1 on, causing sounder loop current to flow, the voltage across Q1 would be nearly zero and thus the voltage across R5  would be nearly zero (some small voltage would be across R5 due to the voltage drop across R10) and the positive op-amp input would be near zero, which is the same condition as when the key is open.  So the computer would not know the correct state of the telegraph key when Q1 was conducting.

Now consider the shunt removed from D5 and D6, so that they are operational.  With D5 and D6 returned to the circuit, and with the key closed and Q1 conducting, the voltage across R5 is now at least +1.4 volts due to the voltage drop across D5 and D6. That +1.4 volt input is greater than the +0.7 volts on the negative op-amp input, which causes the output of the op-amp (signal lead DSR) to be near the positive supply voltage, +6VDC, telling the computer that the key is closed.  With D5 and D6 in circuit as shown on the schematic, the DSR signal now reflects the state of the key regardless of whether Q1 is on or off.

Figure 1 – RS=-232 Version

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Photo of RS-232 Prototype




The TTL version of the interface is similar to that of the RS-232. The major changes are that the DSR and RTS signals need to be inverted with respect to the signal levels for the RS-232 version, as provided by Q2 and Q5. Also the power source is +5V as provided by the USB to TTL converter and the return is the USB ground signal. Referring to the schematic below, you will see a number of generic 2N3904 NPN transistors. In quantity the price of these transistors is less than one cent each, so I often use them without regard to cost. Q5, R10 and R11 take the place of the op-amp with a switching transition threshold of approximately 1.3 volts at room temperature. An op-amp would provide tighter switching threshold range, but I wanted to try a circuit without an op-amp. Q3 and Q4 form a darlington driver for the LED. I used this configuration so that there was as little loading as possible on the collector of Q2, so that when the voltage to the MOSFET Q1 was intended to be high, it would be as close to +5 as possible. This is more of a concern when IRF type MOSFETS are used instead of the IRL MOSFETs. The IRF type MOSFETS would like a gate voltage of at least 4.5 volts for the on state, while the IRL MOSFETS are happy with 2.5 volts for the on state.

Figure 2 – TTL Version
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[1] If you use an RS-232 serial card installed internally in your computer, rather than a USB to RS-232 cable, you may find that the signal voltages are +12VDC and -12VDC rather than +6VDC and -6VDC as found on the USB to RS-232 cable.  The MorseKOB interface should work with either system voltage.

[2] In this document I use the convention of “electrical current” flowing from a positive electrode to a negative electrode.  Some readers may be more familiar with the convention of “electron flow” from a negative electrode to a positive electrode.  Both perspectives are correct.