Tachometer Driver Circuit - LM2917N

Thread Starter

Zaloryan

Joined Jan 26, 2019
3
Salutations,

I'm looking for assistance in troubleshooting my first circuit board based project.

Project Goal:
The tachometer was completely unresponsive in my old pickup truck. I would like to retain the look of the original instrumentation and took to find some sample circuits as a basis to begin. The truck has a 4 cylinder engine and the ignition is electronic with a General Motors type High-Energy-Ignition (HEI) trigger. I found a sample circuit in a booklet from Texas Instruments for a Gasoline Engine Tachometer which calls for IC LM2917. Please see attached image for the sample circuit, 'TI-Tach Schematic'.

The Bill of Materials I've used is as follows:
1. ‎Zener Diode 20V 1.3W DO41 (Quantity 1) BZX85B20-TAPGICT-ND‎
2. Trimmer 500ohm 0.5W PC (Quantity 1) 3386P-501LF-ND
3. IC Converter Fq to Hz 14DIP (Quantity 1) LM2917N/NOPB-ND
4. Res 10ohm 0.5W 1% (Quantity 1) BC4399CT-ND
5. Res 470ohm 0.5W 1% (Quantity 1) BC4173CT-ND
6. Res 10kohm 0.5W 1% (Quantity 3) BC4393CT-ND
7. Res 20kohm 0.5W 1% (Quantity 1) BC4405CT-ND
8. Res 100kohm 0.5W 1% (Quantity 1) BC4394CT-ND
9. Cap 0.022uF Radial (Quantity 2) 399-4398-ND
10. Cap 1uF Axial (Quantity 1) 399-13735-1-ND

Everything soldered together quite nicely. I have verified that supply voltage to the tachometer is 12volts (~14v when engine is running) and the ground is OK. The signal wire is connected to the negative terminal of the ignition coil. The tachometer needle will respond when battery voltage is supplied, but the tachometer will not respond to any engine RPM variations or display any engine RPM signal whatsoever. I did experiment with the variable resistor (500ohm Trimmer) and the tachometer needle's resting point will change accordingly when supply voltage is applied.

To see my circuit's layout on my PCB, please see the below attachment, 'Tach PCB Schematic'. I've followed continuity from component to component and cannot find anything obvious that may be preventing the tachometer from operating. If anyone can volunteer any advice or share their experience I would sincerely appreciate your contribution. Thank you for your time!
 

Attachments

LesJones

Joined Jan 8, 2017
2,326
Try connecting the signal input to the other side of the coil. As it is electronic ignition it could be the positive side of the coil that is pulsed.

Les.
 

Alec_t

Joined Sep 17, 2013
10,385
The absolute maximum tach input voltage for that IC is rated as 28V, so depending where you take the input from in the HEI circuit you may have exceeded that limit and cooked the IC. Ignition coil primary voltages can get pretty high when the coil current switches off.
 

Thread Starter

Zaloryan

Joined Jan 26, 2019
3
LesJones & Alec_t,

Thanks for your suggestions!

I am concerned about swapping the signal input to the other side of the coil because Terminal C of the 4 pin module is typically used as the trigger for most tachs. Please see attachment 'HEI Schematic'. Additionally, the tach trigger circuit touches the ground circuit in parallel on my tachometer driver and I am concerned that may fry it for sure.

Can you provide any diagnosis steps that I could use to verify if the IC is still OK?

Additionally, I found a proposed Tach Filter Schematic using a 555 IC. See attachment, "Tach Filter Schematic". Do you suppose this would function as an appropriate filter to limit the input voltage for my tachometer driver?
 

Attachments

LesJones

Joined Jan 8, 2017
2,326
If the positive 12 volt feed goes directly to the positive terminal on the coil then the negative terminal is the correct place to get the tacho pulses. When you post a question it is better to give all the information that you have to avoid us having to guess how things are connected. Can you see the LED on the LM555 output flickering when the engine is running slowly ? What value capacitor are you using between the LM555 output and the 10K resistor on the LM2917 input ?

Les.
 

Alec_t

Joined Sep 17, 2013
10,385
The circuit in the post #1 attachment was apparently designed for a pickup of the variable reluctance type. Such a pickup provides an AC signal with peaks going negative as well as positive. If I understand the IC datasheet correctly, such alternating polarity of its input is essential for the IC internal charge pump to work correctly. By taking the IC input from the coil negative terminal, are you actually getting alternating polarity? I guess it depends on what goes on inside the HEI module. You have only 2 caps in your BOM, so I assume you don't have the IC input capacitively coupled to the coil negative?
 
Last edited:

Thread Starter

Zaloryan

Joined Jan 26, 2019
3
Les, I'm sorry that my first post omitted some details.

As for the LM555 filter, I have not yet constructed it. It was a sample circuit that I found on the internet; do you think it will serve as an applicable filter to limit voltage and create a scalable signal for the LM2917 Tachometer Driver? What value of a capacitor would you recommend I use between the LM555 output and the 10K resistor of the LM2917 input? What purpose would this capacitor serve in addition to the LM555 & LM2917?

Alec_t, please pardon my ignorance, could you please educate me on how you identified that the sample #1 attachment was used for a variable reluctance pickup?

I have 3 capacitors used in my BOM for the tachometer driver; 1 of them is 0.02uF capacitor wired in parallel to the 10K & 20K ohm resistors that receive the ignition coil input, then go to Pin #1 of the LM2917. If you view 'TI - Tach Schematic' in Post #1, you can see the capacitor. Would this qualify as a 'capacitively coupled input' to the LM2917 IC?
 

LesJones

Joined Jan 8, 2017
2,326
The reason I asked about the capacitor value on the input was because I assumed that you were using the 555 circuit. This would give a pulse output from pin 3 with a low level of zero volts and a high level of about +15 volts. If had not used an input capacitor this voltage swing would not have crossed the input threshold of zero volts. With a capacitor it would have shifted the DC offset of the voltage negative by half the average DC value of the waveform so it would then cross the zero volts threshold. If the capacitor was a very low value (Say a few pF) then the signal would probably have been attenuated too much. The circuit on page 17 of the data sheet raises the input threshold level to about + 0.7 volts by connecting the inverting input to about +0.7 volts using a forward biased diode. This removes the need for a capacitor on the input.

Les.
 

Alec_t

Joined Sep 17, 2013
10,385
Alec_t, please pardon my ignorance, could you please educate me on how you identified that the sample #1 attachment was used for a variable reluctance pickup?
My bad. I misinterpreted the schematic and forgot about the anonymous cap at the connection to the coil. Your BOM doesn't include that cap, but it would ensure alternating polarity input to the tach even if the HEI output is uni-polar.
 

shortbus

Joined Sep 30, 2009
6,988
Aren't all of the circuits posted so far for points distributors? And don't think the modern car/truck tachs will work with a standalone circuit like those posted. They are expecting some sort of signal coming from the ECM. Many of the modern tachs are not the old type D'Arsonval type meter/movements but a stepper motor movement like the other gauges.
 
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