Basic ATMega8 based tachometer

This is a very simple replacement board for cheap Hall effect sensor tachometers.

These are horrible and unrealiable: refresh rate is horribly slow and it is very sensitive to noise; as soon as I switch my lathe's VFD on, it showed random numbers; hopefully this version will behave a bit better.

It very basic but it allows to configure via the 2 push buttons:

• the number of magnets,
• the refresh rate (in steps of 100ms).

The configuration is stored in the EEPROM. To enter the config mode, push the PB_0 button. It will show the current value of the number of magnets which can be increased with the second button (PB_1). Values will rotate up to 10.

A second push on PB_0 will validate the change and allow to set the refresh rate. Similarly, each click on the second button will increase the refresh rate by 100ms (up to 2000 before it goes back to 100).

A third click on the PB_0 button will activate the new values, store them in the EEPROM ang return to normal (rpm counting) operation.

During normal operation, is no signal is detected, "----" will be displayed.

Original board

The original board is very simple and uses an unknown microcontroller, so it cannot be hacked easily.

Replacement board

The new design is basically the same as the original board but using an ATMega8 as microcontroller so it's possible to write an half-decent firmware.

I chose the ATMega8 because I still have a few of them and it good enough for such a simple project. But really, any Arduino clone would do the job.

The protoype has been made using a protoboard and the schematic is the bare minimal:

• a 7805 and a couple of caps (the ones from the original board),
• a 16MHz cristal + 2x20pF caps,
• the ATMega8,
• the 4x 7 segment display (common anode) from the original board,
• a few headers and connectors,
• a pair of push buttons.

Once installed, compared to the original module, it works much better (fast response time and no issue with the VFD and motor noise).

The code of the firmware (src/tacho.ino) is licensed under the GPLv3.