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I found the below schematic here to while trying to build a tachometer for my motorcycle.

schematic for my tachometer

  • What does the circuit element labeled "BOARD" mean (directly left of the 22pF capacitor) - is it just a wire connection?

  • Does the 5V terminal above the NE555 timer go the \$V_{cc}\$ of the IC? Otherwise, it seems to go straight to ground (wouldn't that be a short-circuit?)

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  • \$\begingroup\$ "Board" is a printed-circuit capacitor. Apparently, it has a breakdown voltage high enough to prevent arc-over. \$\endgroup\$
    – glen_geek
    Commented Jun 19, 2017 at 14:09
  • \$\begingroup\$ maybe you'd want to link to the source of this. I literally can't find a worse way of doing this than what this proposes. capacitively coupling in the ignition line – OK, I'd throw in a series resistor, but whatever. And then the whole unfiltered 555 circuit – urgh. If you need to count events, use a counter; many microcontrollers come with that. I refuse to explain a bad circuit. \$\endgroup\$
    – Marcus Müller
    Commented Jun 19, 2017 at 15:00
  • \$\begingroup\$ @MarcusMüller here it is:sportdevices.com/rpm_readings/index.htm \$\endgroup\$
    – Andrei Grigore
    Commented Jun 19, 2017 at 15:01
  • \$\begingroup\$ I don't know how to do this... i tried hooking up to the 12v part of the ignition and use a voltage divider to get approx. 5v and then used an Arduino to count how many times it gets a digital HIGH in a second but now i realize that it was a mistake as i should have measured the period between two consecutive digital HIGH signals... somebody told me it was a bad idea to use resistors in parallel with the coil. It sort of worked. I got some readings but most were aberrant. I thought of using a "simple" inductive pickup... a few loops of wire around the ignition wire and an arduino to read data. \$\endgroup\$
    – Andrei Grigore
    Commented Jun 19, 2017 at 15:06
  • \$\begingroup\$ Of course, being careful not to get a voltage that is too high, and using a Zenner diode to separate voltages above 5v and then stepping them down a bit in order not to damage the Arduino... i could just use an AnalogRead as it doesn't have to be very precise. It just has to be above 3-4 volts so that it would not be a false positive \$\endgroup\$
    – Andrei Grigore
    Commented Jun 19, 2017 at 15:09

2 Answers 2

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From the description, you have to form a capacitor using the PCB. One plate on the top layer and one plate on the bottom layer (same dimensions and position).

You connect one plate to your pickup clamp and the other to the 22 pF capacitor.

Now don't ask me for dimensions of the plates. I also imagine that the PCB thickness is important as well. Maybe even critical to have a minimum strength to prevent arc-over (in which case you also need enough clearance around that plate). If you would know the required capacitance you can calculate the area of the plates, but otherwise it's pretty much guessing. I'd probably start with finger-print sized plates for no obvious reason.

Your second part is guessed correctly. The 5 V goes to Vcc of the NE555. The schematic follows the custom to draw Vcc at the top and GND at the bottom.

Still having unlabeled pins I'd consider bad style (imagine an inverting circuit where voltages go below zero and GND is suddenly on the top and stuff).

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  • \$\begingroup\$ I understood the second part perfectly, but the first part.... why is that kind of "capacitor" needed? What metal am i supposed to use? I was so happy when i first saw this circuit because it seemed really simple and easy to build... that "plate" makes it very difficult now...can i just skip it? It is noted with an asterisk around so i figured it is optional. I just can't see the purpose of it (well... i m not an electronist... i don t see the purpose of some other parts in this crcuit aswell but at least those aren't bugging me...) \$\endgroup\$
    – Andrei Grigore
    Commented Jun 19, 2017 at 14:45
  • \$\begingroup\$ A capacitor is really just two conductors with an isolator in between. Google "plate capacitor". You can build a plate capacitor by having one plate being the PCB copper layer on top of the PCB, and the other one on the bottom, like Arsenal explained. \$\endgroup\$
    – Marcus Müller
    Commented Jun 19, 2017 at 15:01
  • \$\begingroup\$ @AndreiGrigore I'm not entirely sure why it is needed. My guess is like glen_geek suggests to prevent high voltage arcs. Now you could go and replace that with a high voltage rating capacitor. But FR4 has a breakdown voltage of around 20 kV/mm so a typical FR4 PCB would have a 30 kV rating, and finding a capacitor with that voltage is a bit difficult and very costly. Probably a smaller rating would be enough, but I can't say what would be a safe value. If you feel lucky you could try a 3 kV 10 to 22 pF ceramic capacitor as replacement. But be ready that magic smoke happens. \$\endgroup\$
    – Arsenal
    Commented Jun 19, 2017 at 15:30
  • \$\begingroup\$ I suspect the asterisk doesn't mean optional, but was a footnote explaining the construction or specs of the PCB capacitor. I assume the footnote was omitted when the schematic was copied from its original context. \$\endgroup\$
    – Evan
    Commented Jun 19, 2017 at 16:21
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First of all you have mentioned you are trying to build a tachometer, but from what I understand this circuit looks more like an ignition system circuit. I may be wrong here.

Now about the board - it refers to the board (PCB) capacitance. And Yes the 5V goes to the VCC of the 555 timer IC.

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  • \$\begingroup\$ Well it picks up the ignition signals to measure the engines RPM I guess. With a known tire size and gear ratio, you can even calculate the speed of the wheel (not always the same as the speed of the vehicle). \$\endgroup\$
    – Arsenal
    Commented Jun 19, 2017 at 16:31
  • \$\begingroup\$ "... but from what I understand this circuit looks more like an ignition system circuit. I may be wrong here." Yes, you are wrong. The clamp reads ignition pulses by capacitive pickup through the spark lead insulation. "... the board - it refers to the board (PCB) capacitance ..." doesn't really explain anything. The OP is wondering how to make it. You haven't addressed this. Expand your answer some more to improve it. \$\endgroup\$
    – Transistor
    Commented Jun 19, 2017 at 21:04

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