Digital input for analog tachometer circuit

Thread Starter


Joined Nov 30, 2015
I'm looking to build a much better tachometer circuit for my dads Torino. I did a very basic circuit that uses no transistors but it's slow, not very accurate and kinda sucks and I had to modify it to even work right. I want a good circuit but can't find anything online. It has a MSD 6 ignition installed. I'm using the digital output signal from it for this. I will at some point take my oscilloscope to that and see what pulses per minute and rpm there is. For now assume mutilple pluses per engine cylinder as its like a hot fire multiple sparks per cylinder for cleaner burn? I think I read that online. So I need it to take say 3 pluses per cylinder and like 4 cylinders per rotation. I need it to go to a 1ma needle. It doesn't need to be the best just good, small and easy to do. That way it can fit inside the tight tachometer casing. It's a rare and valuable ribbon tachometer so I need to take care. They cost $400 untested! Insane.

I've seen those 555 circuits and thought about it but they're all taking their inputs from the coil! I don't have that option here. And I can't find a circuit with a digital input. Only ever the coil. So if I can get some help that would be awesome. I will go through my IC chips I have and put a list here. I know some will be way overkill but what else I'm a going to use them on? I'm more into vacuum tubes and doubt I'll build much else with them. So I'll list them later tonight and see about getting the MSD digital output captured on my oscilloscope. I upgraded that so now it's only 20+ years old instead of 55+ years old. It's a Tektronix 2232, old was a Tektronix Type 317.

So for now assume 3 pluses per cylinder until I see otherwise. MSD is so kind to to give the info on their digital output signal. If only they would. That would make this easier starting off. Well at least that part will only take a couple minutes and guessing an rpm of 750 idle that will give a starting point for this. I believe it's a 5v p-p signal from it. I'll confirm later tonight or this weekend at latest. Thanks


Joined Jun 22, 2012
Lm2907 chip is an excellent rpm counter, used it loads of times, you can use an optical or magnetic pick up from the crank shaft case, or points pick up.

Thread Starter


Joined Nov 30, 2015
My reading of the MSD Digital 6 Plus (PN6520) Installation Instructions is that the tacho output of the unit is a 12V pulse with a repetition frequency equal to the points-switch/pickup frequency, so won't be 3 pulses per cylinder. The number of sparks per cylinder firing is variable, so not strictly proportional to rpm.
If I'm right it will be 1 pulse per cylinder at 12vish with I think 4 cylinders per revolution, so maybe 4 pulses on the digital port per revolution? When I tested it quite awhile ago when making the first circuit I used a analog multimeter which doesn't change with frequency. It didn't show more then 5v. Maybe you found the MSD 6a. This model is older and is only the 6. I may need to do a lot of research finding it. Give me a bit and I'll post my IC chips list then I'll start research on the MSD.

Thread Starter


Joined Nov 30, 2015
Here are my IC chips. I'm not familiar to IC chips and their codes. Hoping some of these will work for this. And hoping none have to be programmed. I don't have any means to do that. The x2,x4 etc means I have 2 and 4 chips of that same number. No x means jut one chip.

EF682IP (Large chip)
ADC0809CCN (Large chip)
GM76C88AL-12 (Large chip)
4116R-001 (X2)
7474N B (X4)
SN74LS76AN (X4)
SN7402N (X2)
SN74S194N (X4)
T74LS20B1 (X2)
T74LS112B1 (X2)
UCY 74121 (X2)
UCY 7483

UCY 7475
SN7486N (X2)
M74LS75P (X2)
7476 (X2)
DAC0808LCN (X2)
SN7408N (X2)
HD74LS30P (X3)
74LS27 DC (X2)
7404PC (X2)
7400 (X2)


Joined Jul 1, 2009
MSD = Multiple Spark Discharge for those who might not know (reading this thread). In an MSD6, you have a tachometer output. What makes you think it will output a pulse for every spark discharge? Discharging a spark plug multiple times in a specific pattern to assist fuel burn in a single cylinder does not necessarily equate to them outputting multiple tach pulses. From an engineering standpoint-- you wouldn't. The whole point is to make the fact that you are firing multiple times into a cylinder as transparent as possible-- so you'd still only output a single pulse per cylinder on the tachometer.

I might be wrong, but I've looked at the MSD6 documentation and schematics and all kinds of things-- and one pulse per firing (whether that firing is one spark discharge or multiple per cylinder) makes the most sense. Otherwise you wouldn't need a separate tach output-- you could just use any clamp on, hall-effect method and measure those discrete pulses directly.

Tachometer measures RPM-- you'll only measure off of the #1 cylinder in any event-- and you only need to know when it fires once to register one revolution of the crankshaft.



Joined Sep 17, 2013
The 7474 that you have could be used for a monostable as the basis of a tachometer, but would need quite a lot of ancillary passive components and tweaking to make a half-decent tacho. You would probably be better off using a LM2917, which is a chip intended for this purpose and has a built-in voltage stabilizer for greater accuracy.