My converted Trix 22805 PA-PA has a DCC decoder with a single output for the front lights and a single output for the rear lights, at about the value of the rectified track voltage. Thus, headlight, numberboards and mars light are either all on or all off.

The mars light is made to blink (4 spikes of about 10 ms in 2 seconds on a weak background) by the following circuit, made up of [partially unknown] discrete components:



The LED is constant-on at a low level, via the 10k resistor. I assume the blink is made to happen by a breakdown in components B and C so that the LED is connected to minus via the 1k resistor as well; at the same time, the capacitor unloads partially.

The voltage across the capacitor is fairly independent of the source voltage, at about 2.5V, which makes me guess component D is a zener diode.

The voltage across component A is in the region of 1 V; a diode that prevents the capacitor from unloading across the source?

The physical appearance of components B, C and D is shown in the pictures (scans, actually, so that the focus is wrong in the second one) below.

On the first photo (of the pcb with the lights), Component B is orange/green, and appears here below the 10k resistor:



On the second photo (of the "blinking" sub-pcb on the pcb connecting the outputs of the decoder in the "main" PA to the other PA):
Component A is unidentifiable, as it is located under the capacitor and additionally is covered with a black (isolating) substance.
Component C is black, with the number 1 in white (to the left of the capacitor).
Component D is orange, with a black band (above the 1k resistor).



I would be interested to know the type of components A through D, and ideally, how to simulate them in LTspice.

Any help would be greatly appreciated.

On further search I found an old web page that looks familiar to the circuit above …

Jeroen

Edited by user 10 September 2018 22:44:45(UTC)  | Reason: Correction in description and addition