Please help rectify the errors in this digital speedometer circuit.

Thread Starter


Joined Oct 12, 2009

I am helping my friends with this circuits to complete a college project. The site claims this to be a tested and proven circuit. But the supply pins of nand gate and IC 556 are not connected properly. Well that can be rectified. But the circuit does not works when done on bread board or any. I just get the count "00" after doing possible changes from our side.

Please help me with this. Or if i could get a different speedometer circuit that would be of great help to us.

Thanks in advance.


There's something else unusual on that circuit: NAND N4 output is tied to 5V. I'm not sure whether that's intended. Actually, since IC4 (decade counter) pin 14 is the input, it's almost certainly not intended. I'd start out by disconnecting 5V from IC4/14 and N4 out.

Then you just have to start chasing the signals through. Do you have an oscilloscope? That will make it really easy. However, even a simple LED or audio probe that indicates when a signal is high or low will probably be enough.

Make sure you're getting a signal out as you turn the wheel, then make sure you're getting clocks & pulses & so forth at other appropriate points. Figure out how the circuit is supposed to work, then figure out what sort of signals you'd expect to see if it was working, then figure out what's wrong with the signals you do see.

Thread Starter


Joined Oct 12, 2009
after disconnecting 5v from pin 14 and 11 of IC4 and N4 respectively.. 5v should be connected to pin no 14 of N4, N5 and IC1(a)..

also the pin connections of IC 8 & 9 make me confuse (pin no 3,4,5 & 6).

I am getting only "00" and nothing more. The pulse from LM324 seems like it has no effect over the whole circuit.
Do you understand how the circuit is supposed to work? I didn't read the text very carefully, but I think this is the general idea:

  1. Wheel sensor puts out pulses when wheel turns
  2. Decade counters count pulses
  3. Every half-second or so, the decade counter outputs are transferred to the latches, and the decade counters are reset.
  4. The latch outputs go to the BCD-7Seg converter and then to the display.
If you're getting wheel-sensor pulses, the next thing to do is see if the decade counters are counting up. (They might be getting reset too often, or not counting or something.) Next, check that the latches are copying out the counter values as expected. Finally, check the BCD-7Seg stuff.