Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Car: Connected microcontroller-based tach - dash tach quits

Status
Not open for further replies.

Scruit

New Member
I am designing a device that need to know the rpm if the car very precisely and instantly. I have a Z8 microcontroller and I wrote a simple routine to count the number of milliseconds between each pulse of the dashboard tach signal wire, and then display the calculated RPM on an LED numerical readout.

The I am using a standard IO port and counting on the leading edge of the tach pulse. low-to-high signals to compute rpm based on the current ms timer, then reset the timer. The input pin is wired up using a 470ohm resistor to pull down to vehicle ground and the tach signal wite connected directly to the same input pin.

When I hooked up the device to the tach signal wire I noticed that the RPM readout failed to 'see' the rpm pulses. I removed the pull-down resistor and it started working, but it read only half the correct RPM (but that could be my computations) and that the dashboard tach quit.

When I disconnected my device, the dashboard tach started working again.

What would you guys suggest? I was thinking about a much bigger pulldown resistor, like 10k or something.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Why do you need a pulldown resistor at all?. The existing tacho works, so presumably the signal is perfectly acceptable for that. I would suggest you use a scope on the signal to find out exactly what it is, then use an opamp as a buffer/amplifier to feed your Z80 circuit.
 

Scruit

New Member
Nigel Goodwin said:
Why do you need a pulldown resistor at all?. The existing tacho works, so presumably the signal is perfectly acceptable for that. I would suggest you use a scope on the signal to find out exactly what it is, then use an opamp as a buffer/amplifier to feed your Z80 circuit.

If I use the 470 Ohm pulldown resistor, the dash tach works but my project tach doesn't.

If I use no pulldown resistor then the dash tach stops working and my project tach works.

I need to get hold of a scope... :?
 
Status
Not open for further replies.

Latest threads

Top