Modifying $3 DMM (Generic 838) for extremely fast DC/Ohm response.

[DavidAlfa]

My brother's car died, the control unit in the steering wheel was taken over by some angry gremlins, it's turning random stuff on,  like wipers, lights, not recognizing the key and making incomplete beeps, like it was rebooting or something.

As I don't have any portable scope to watch the voltages, and a DMM is too slow to catch spikes/drops, I thought on modifying a cheap AliExpress DMM I threw in the boot some time ago for emergencies.

Basically:
- Reduce the input RC integrator.
It's made of 1M + 100nF, swap to 1nF so it allows much faster response.

- Increase the RC oscillator frequency to have more readings.
It's 100K + ?pF, running at 48KHz. Replaced with 22K resistor, increased  to about 120KHz, giving a lot more readings per second.

Now it can detect square signals up to 25Hz.
LCD update rate is about 10Hz.

Not that I can see a value changing so fast, it's about seeing something is wrong there.
If the voltage is steady you'll get a fast changing decimal.

Also this provides extremely fast "beep" response, unlike before where it took 1/2 sec.

Downsides: AC voltage is useless as the integrator no longer filters properly. Not a problem in a car.