Help with ne555 pins for capacitive ignition pickup.

Hi there!
This is my first post.
I’m building a dyno for mopeds/motorbikes and are going to use an arduino uno together with Simple Dyno software.
I need a way to pick up engine rpm and found this circuit:

My problem is the pins of the ne555 in the circuit I found is not named correctly.

Pin diagram for ne555:

Here’s the pins I’m pretty confident about:

Pin 1(gnd) - Ground
Pin 3(output) - Out (to arduino)
Pin 7(discharge) - DIS
Pin 8(+Vcc) - +5v

and the one’s I’m not confident about at all:

Pin 2(trigger) - IN-?
Pin 4(reset) - not connected?
Pin 5(control voltage) - not connected?
Pin 6(threshold) - IN+?

If there’s anything else with the circuit worth commenting on, please feel free to do so.

I’ve just started learning electronics and hope to in time be able to figure out stuff like this myself but at the moment I’m really eager to get this dyno up and running!


Picture of the dyno project attached.

See image:

Thanks for the reply!

I must admit I'm still too dumb for that diagram of the 555 to really help any...

In the diagram I posted, is my assumption that pin 2 is marked "IN-", pin 6 "IN+", and pin 4 and 5 are not connected to anything right?

That's what they show.

Ok, now I get it; Pin2 is connected to "-" on one comparator circuit and pin6 is "+" on the other. That's why they marked it like that "IN+/-".

No problem in keeping pin 4 and 5 open?

Thanks again!

4 and 5 are used for specific applications.


Thank you great sir!

Hi You can place a 0.1uF cap from pin 5 to gnd, and one from pin1 to pin8 as close to the LM555 as possible. This will help with noise immunity.

Tom.... :)

Thanks for the advice!
Short explanation of why those caps help reduce noise?

Hi, You can consider noise as unwanted AC, in the environment you will be in, you will have electrical interference induced onto the power supply wires. The capacitor begins to conduct electricity as the frequency of a voltage across it increases, DC current does not flow but AC does. The cap shorts the noise out so it does not pass to the LM555. The cap on pin 5 is there because the LM555 uses a potential divider inside it to work with the comparator, part of the divider is connected to pin 5. This divider is also sensitive to noise.

The best place for the capacitive pickup is near the plug end of the lead, not the distributor/coil as this is where all the firing signals originate.

Tom.... :)

Thanks for that great explanation!

Tom, Does reset pin 4 have to be connected to pos rail in this instance.(as suggested below)

Hi, Yes, pull it to supply, if its left open it may stop the LM555 from toggling output.

Tom..... :)

bluejets: Tom, Does reset pin 4 have to be connected to pos rail in this instance.(as suggested below)

Great link!

So, I built the circuit but I'm not getting it to work. I had wired the bt149 backwards, and thought that was the problem, but guess not.


  1. Can the backwards wired thyristor have caused any damage to itself or the 555?

  2. My pickup wire is about 4m long shielded microphone cable - too long?

I tried with and without the shield connected to arduino ground, no difference. I tried both twisting the wire(shield and outer insulation stripped off)around the ignition lead and using a metal clamp.

I've gone trough the circuit multiple times to check for errors, can't find anything wrong.

Here’s a picture of my board. Vertical copper strips, cut under the 555 otherwise untouched, no jumpers on the underside.
Anything obviously wrong?

Hi, I can see pin1 of the LM555 and the cathode of the SCR connected together, but not connected to gnd.

Do you have a DMM to measure electrical parameters?

Tom.... :) PS, I would advocate IC Sockets in first try/prototype circuits.

Thanks for the reply! There is a jumper from pin 1 to ground, I bent the bt149 to show where it's pins are connected, the end of the jumper is hiding under it.

Yep, got a multimeter. You are right, I'll order some sockets!

Hi, Have you got the gnd of the circuit connected to the gnd of the arduino?

Also if you jumper the gate of the SCR to the 5V positive, it should trigger the SCR. Use your DMM to see if pin2 of LM555 goes to gnd when you connect gate to 5V.

Tom.... :)

Yep, connected to the arduino ground. Thanks, I'll test it tomorrow and report back.