# Junkyard scavenging nets a tachometer to play with

We never thought to hit the automotive junkyard to find electronics we could play with. But [Istimat] was able to pull this working tachometer from an otherwise destroyed motorcycle dashboard. The Kawasaki part has just three pins on the back of it. By connecting 12V to the IGN pin, ground to GND, and tapping a 12V wire on the unlabeled pin he was able to make the needle dance and knew he was getting somewhere.

His microcontroller of choice for the project is an Arduino board. But the 5V logic levels aren’t going to put out the square wave needed to drive the device. A search of the internet led him to a 2-transistor circuit which lets him get the results seen in the video. His plan is to add functionality that uses the Arduino to pull data in from just about any source and display it on the dial. That computer desk that featured all the CPU load readouts immediately comes to mind.

Do you think the square wave circuit is more complicated than necessary? Could this be done with just one NPN transistor and a pair of resistors?

## 13 thoughts on “Junkyard scavenging nets a tachometer to play with”

1. philwatcher says:

Use one NPN, a base resistor and a pullup, invert the signal in the arduino, and you have your 12V signal.

2. KNfLrPn says:

Whether it could be done with one transistor depends on the requirements of the input. The circuit he used allows for a decent current to flow in to the 12V input. A single NPN and resistor would only work if the input requires almost no current.

1. philwatcher says:

The device has 12V input, that is why I assume no significant amount of current will flow through there. But you are correct, the “proper” way would be the two-transistor circuit.

If you had an open-collector output, you could use one transistor (with high currents), but then again that would essentially be the same circuit as above.

2. Dax says:

It’s unlikely to be very low impendance.

Besides, did he even try if it works with 5 volts?

1. I tried it with 5v and it didn’t seem to work…

3. DrAvIn says:

want some fun .. with alot of patient wiring and a bit of coffee the digital dash set from a 90’s mercury villager mini van looks nice monitering all that needs to be seen on a good desktop system ….. fuel moniter section is nice for keeping track of that pesky battery backup too lmao

4. DrAvIn says:

OH!!! the motors and air pump (lumbar support) from the same van are fun little toys too .. used them many times on builds .. most rescent is a 12V kids deere gator with electric dump and a poweer snowplow lmao

5. DrAvIn says:

… a tach requires something to trigger it … normaly it would be controlled by the opening and closeing of the ignition points (oldschool engines) they work ok monitering anything with moveing parts if a small momentary button is connected and there is something similar to the lube on a camshaft to press the button … modern electric ignition is a bit more complecated but the same effect can be replicated but keep in mind they arent actualy useing the 12V from the chargeing system they use the much more painfull levels burning through the coil (it’ll knock a person on thier butt)

6. Cars and bikes are full of electronics and really useful actuators to play with. Unfortunately most of the microcontrollers you find are OTP and can’t be reprogrammed, but the drivers they connect to certainly can be.

7. dyno says:

Lots of cool stuff in the automotive junkyard. My neatest find was a qualcomm omnitrack that came off of a big rig truck cab, see here: http://w6iee.blogspot.com/2012/01/microwave-radio-parts-at-unlikely-place.html

Its mainly got ham radio uses, its got an excellent and well documented PLL synthesizer thats has been widely used as a microwave-band transverter local oscillators, and its also got a 1-watt power amplifier that can be moved to X-band with minimal effort.