I've built an ignition timing light, following: http://citroen.tramontana.co.hu/en/ignition/stroboscopic-timing-light

here the original schematic:

schematic diagram

I've changed some components:

  1. Instead of the BD135 BJT, I've used a BD128D.
  2. Instead of R3 (4M) I've used a 3.9M R
  3. as LED1 I've used a Nichia NFSL757GT LED
  4. as setting current R5, i've used two 220ohm R in parallel
  5. Instead of a 240ohm R (R1) I've used a 220ohm R

Here the schematic as modified:

modified schematic

When I apply it to the spark plug HT lead, the LED flickers as it has to do, but it is barely switched on.

The trigger cable is connected to the spark plug lead insulation via a crocodile clip and it collects the signal via capacitive coupling with the cable.

  • The cable insulation is made of PVC (dielectric constant 3.19) and is 3mm thick.
  • The inner cable is made of copper 1mm thick.
  • The voltage provided by the ignition coil is 14 000 V.
  • There is about 1000 spark each minute.
  • The crocodile clip is 5mm wide.

The trigger cable is an audio cable, which I believe is shielded. I've connected the shielding to the battery - pole)

I think that maybe the trigger signal collected is not enough to open the thyristor and the transistor.

Is it possible to calculate the capacitive coupling?

I would like to try to simulate the trigger signal with a Keithley power supply. Which current and voltage do I have to set, in order not to destroy nothing?

  • 2
    \$\begingroup\$ 1)Did you notice the part that says, "Using a shielded cable for the trigger input line is essential to avoid false input from the other cylinders."? If you haven't done that, you'll be switching on the LED when any cylinder fires, which means the supply capacitor won't charge very well. 2) What LED (exactly) are you using? 3) Just how bright is "barely switched on"? This sort of strobe won't be very bright due to the short drive pulses, so you need to take that into account. \$\endgroup\$ – WhatRoughBeast Sep 15 '18 at 11:41
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    \$\begingroup\$ What LED did you use? Link please. \$\endgroup\$ – Andy aka Sep 15 '18 at 11:44
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    \$\begingroup\$ Closing as unclear until you show the schematic of exactly what you built. No, I'm not going to keep scanning back and forth between the schematic and your text description of what the part values really are. \$\endgroup\$ – Olin Lathrop Sep 15 '18 at 12:43
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    \$\begingroup\$ updated,I've put both the schematic, hope now is more clear! \$\endgroup\$ – Luigi Sep 15 '18 at 13:16
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    \$\begingroup\$ That circuit doesn't make much sense for direct connection to the spark plug lead. Is TRIG really coming from a magnetic pickup, perhaps? What exactly is TRIG connected to? \$\endgroup\$ – Olin Lathrop Sep 15 '18 at 13:31

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