View Full Version : Somewhat OT, but useful: TigTac Transistor Ignition and Tach with PIC and LCD

02-04-2015, 02:09 AM
As a project on MEM I have been working on a combination Transistor Ignition and Tachometer mostly for model ICEs, but it could also be used for any other ICE that uses a conventional ignition with points or inductive/magnetic sensor, and it could also be used just as a tachometer and dwell meter. This also shows how to use an inexpensive 14 pin PIC16F1825 (or similar device with MSSP and I2C), with a cheap $2 Chinese I2C-LCD adapter and a standard cheap 16x2 to 20x4 LCD module. Here is the schematic:

This is the I2C module:

Good tutorials and info on I2C communication with Microchip PICs and the Arduino are:

I made a short video clip of the device showing how it reads dwell in mSec as well as RPM (based on time between cycles), simulating the points with a PCB pushbutton:

The source files for the Microchip PIC project are in the zipfile below. I'll be updating it as I progress. You can modify the I2C_PES.c and I2C_PES.h files for different MSSP modules, and the LCD_PES files should work for most 16x2 to 20x4 or 40x4 LCD modules.

I also made a little video of the test setup which has points and condenser from a 1977 Toyota, and a 12V ignition coil connected to a spark plug.

Enjoy! :)

02-04-2015, 08:08 AM
Pretty neat! I especially like the input power protection you did that is so often missing in designs like these.

About a year ago I said to a friend that for one-offs and small quantities, I no longer needed to design any hardware because pretty much anything I needed was already for sale cheaper than I could build it. With the rise of the $3 Arduinos and $5 displays, I keep being proven true. The only piece of hardware I've put on a PC board in the last year is this (http://www.cedarlakeinstruments.com/blog/archives/219). And that was because I couldn't find one that was built the way I wanted it. I probably won't sell too many at that price even with shipping included, but that's OK. It was for my own needs anyway.


02-04-2015, 07:30 PM
Why not just an SSR? They can operate on 5 VDC and maybe 10 mA, and can control a 10 amp or greater load, with isolation.




02-04-2015, 08:35 PM
Couple reasons. It was initially designed to sit on top of a solenoid valve to make it easy to control from the Arduino and that drove the form factor. I wanted the LED for state indication and the design morphed from there.

Also wanted to be able to easily add a fuse without an external fuseholder and a reverse diode for inductive loads. And because most of the "name brand" SSRs with DC output that I could find new were too expensive. By the time I've added the things I want to one of those SSRs you linked, I'm already above my total cost. I can get PC boards made really cheap.

02-05-2015, 09:18 AM
The ignition side of the circuit can be a source of common mode interference from pulsed ground current if special attention isn't paid to the board layout. It's probably worth including in the diagram a chassis ground point for the BDW42G transistor rather than feeding the coil current out the same ground wire as the other circuitry. That was a problem that popped up for me back in the 1970's when I designed a similar ignition circuit. That one used a one-shot monostable driver between the Hall effect sensor and the output transistor, and ground current drove it crazy. A second monostable drove the output for the tach - a simple D`Arsonval movement ammeter, probably 1ma but I don't remember.

A little RFI filtering at VIN also goes a long way to holding down high frequency switching noise inside and outside the chassis. Affordable PIC's have brought a lot to the game since Nixon was in the White House - wish we had Arduino back then :)

02-05-2015, 11:03 AM
Wouldn't a FET be a better choice for an output device than the Darlington (BDW42G) ?

02-05-2015, 11:13 AM
Could this (is this?) be a substitute for a coil and point type ignition for a two cylinder small engine?