Some years ago a neighbour of mine got his hands on a little box, which was marked "Salora Oy Äänimerkkilaite" and it had something to do with old VR trains (VR is a Finnish railway company). His relative has a thing for trains, so he thought that the device might make a good present for him, so he asked me to check what the device does, does it work and would it be possible to turn it into a present.
I'm writing this post because I thought this might be of interest for other train hobbyists also.
Background
The device was a little black box with text "Salora Oy", "Äänimerkkilaite" (sound signaling device) and a serial number. On one end there was several pins for connection to some kind of backplane. In a figure below, the device is shown together with my wires connected to some pins.
Luckily, the device came with a paper which had the schematic and layout, so there was no need to reverse-engineer the device! From the schematic, it was obvious what the device was and how to operate it. Only thing left was to verify whether it works or not.
The device is a sound generator, which creates a familiar "ping-pong-ping" sound which can be heard before announcements are made from the train's loudspeakers. The device only consists of three oscillators creating the tones, a trigger circuit, and an summing amplifier to produce the output signal. Everything is analog. There is also an internal 8.2 V regulator for power supply; the device is intended to run from 15 V DC.
Operation
Here I'll explain the device's operation with annotated schematic. I did this in LTspice, and all the component values are not the same as in the real device to keep simulation time short. For example, some time constants are much shorter in this version.
First, the blocks circled with green are the oscillators. According to the paper schematic, the desired frequencies are, from top to bottom, 659 Hz, 523 Hz and 784 Hz. All the blocks have a trimmer resistor which can be used to fine-tune the frequency. Output of each block is a square wave with the desired frqeuency, going from 0 V to 8.2 V.
The device is triggered with trigger circuits marked with blue squares. Each channel has its own trigger with an RC delay on the left, then two inverters with a hysteresis resistor. Whenever the input goes high, the inverters create a clean rising edge, and input going low, the output also goes low. The trigger circuits are chained so that top one triggers first, and then starts charging the RC delay of the middle one, which finally starts to charge the RC circuit of the last one. The time constant of the delay is roughly one second.
The first, topmost, trigger circuit is triggered from one of the input pins through a 1 k resistor, yellow block. Whenever the signal should not sound, the pin is pulled down. When the pin floats, the trigger circuits operate and a sound is played.
The red blocks are high-pass circuits which create a short, exponentially decaying pulse. The pulse goes high when the device is triggered, and the decays to 0 V with about 1 second time constant.
The exponentially decaying pulse is used as the volume signal for each oscillator. Exponentially decaying waveform gives a bell like sound. The "mixing" is done with two diodes, marked with purple squares.
When the output pulse from the high-pass filter, red squre, is positive, one of the diodes in the mixed (purple square) conducts. When the oscillator (green) is low, it is the diode to the left which conducts and no current flows from the mixer to the output. When the oscillator is high, the left side diode is blocking and current flows through the right side diode. The current amplitude depends on the exponentially decaying voltage of the high pass filter (red). I think this is quite an elegant solution.
The rest of the circuit is just filtering of the square wave outputs of the oscillators and mixing them together with an audio amplifier. Finally, the sum of each oscillator is passed through a DC-blocking capacitor and a potentiometer for setting the volume. This is shown with brown square.
Checking the operation
Now that I knew what the device does, it was time to verify whether it works or not. I attached a laboratory power supply to the power pins, a pushbutton between the trigger and ground and using oscilloscope I probed the oscillators, trigger signals etc., and finally, as everything looked ok, also the output. I saved the output waveform as csv, and on my computer I converted it to wav, and it sounds like this.
As the device seemed to work, I added some circuitry around it. First, because my pushbutton was a normally open, I needed to do a simple transistor inverter for it. This way, when the button is pressed, the trigger is let float and otherwise the trigger input is pulled down by the transistor.
Second, I added a mono audio amplifier circuit and a speaker. The amplifier was very simple, needing only minimal amount of components around it. I think the amplifier was LM386, high voltage version, in DIP-8 case.
Third, I added a DC jack so that I could use an old laptop power brick, outputing 15 V to power the circuit. Here is a picture of the final working device.
Finished device
My neighbor did a nice case for the device, so that he could give it away as a gift. Here is a video showing the case and operation of the device.
