DT-6236C Tachometer Teardown and mod

       I needed a tachometer for my work so I purchased a cheap one. I could not decide if I needed an optical/non contact or contact tach with the spindle so I got one that can do both.

       The one I got online is a model DT-6236C and came with various attachments but no box.

       It works fine except that the laser spot is messy and unfocused so I went to open it to have a look what's inside and see how it was built.

The tach came in this, wrapped in bubblewrap and then inserted in a large envelope.

The accessories it came with.

The contact adapter can be attached permanently with an included screw.

Inside the contact adapter.

The slotted drum cuts the light from an LED to measure speed.

Taking apart the contact adapter.

With the slotted drum removed, we can see that the adapter uses a ball bearing. Nice.

Bearing is a 625Z.

Inside the lens assembly, the LED (right) and photodiode (left) for the contact tach mode.

4 AA batteries are required but these are a very tight fit.

I forced it in and luckily, it worked without breaking anything.

Power consumption at 6V is 90mA in non contact mode, 60mA on any contact mode and 55mA when pushing the memory button. Leaving it alone, standby current is 135uA without it doing anything. Operating current consumption drops when battery voltage decreases so the LEDs and laser diode are run from a resistor and not regulated.

The batteries are a tight fit because the springs are cylindrical. This causes it to have a pretty long compressed length compared to conical springs which can tuck inside itself.

The laser/ non contact mode is ok but the spot is a mess. This is at about 6 inches.

This is about 3 feet away. The farther you go the messier it is.

It doesn't actually matter as long as it works but it looks ugly to me.

Four screws from the back and the front cover is off.

The LED and photodiode for the contact sensor can be seen from the top and bottom of the pic.

The 5mm LED lookalike is a photodiode.

Lens is plastic, no optical coating and has a slight oily residue to it.

I can see the laser diode from here.

Laser diode and photodiode arrangement.

A bunch of 4011 cmos gates.

A couple op amps.

Main uC is an Atmel AT89S5124.

Back of the main PCB

A power connector (barrel jack) is included to use a 6V external adapter (center pos) but you'd still need to hold the button while taking readings.

The power switch is a momentary micro switch on the side. Press it to turn on and take a reading, release and it turns off.

Original placement of the laser diode.

New laser diode position. I just used the next holes closer to the lens.

With the laser diode position changed for better focus, we now get a perfect round spot even at long distances.

Using only masking tape, it can now measure RPM up to a few inches away.

With the provided reflector tape, it can even measure from several feet away.

Now that I'm happy with the new laser diode position, it is hot glued into place for vibration resistance.

Holding the button gets tedious when monitoring machinery RPM so I replaced the switch with a push on push off switch. It does not fit on the original mounts so I used a generious amount of hot melt glue.

       For about $28, this was not the cheapest but works fine after the modifications. LCD is big and clear with a nice white backlight. I have not checked accuracy but I don't think it would be too bad since it uses a crystal oscillator.

Page created and copyright R.Quan ©01 Oct 2015.