Friday, March 13, 2015

@MAKE Electronics Experiment 20: Keypad Security System (Part III--still not working)

This has taken a while. Snow, family visiting, setting up my new generator, an fuel oil spill in my basement, and other events have dominated my attention.

When last we left this, I was convinced that the keypad was sucking current from the circuit. I used this one.  The only videos I've seen of people getting this to work use momentary buttons instead of the keypad, I tried to substitute buttons--that didn't help. Testing various locations with my meter, I see that's not the problem

I checked the voltage coming into the first part of the circuit, and then then through the operation. I put a meter set to continuity connected to the relay. If the upper coil on the relay gets 5V, there should be continuity, and 5V to the lower coil should reset it. As you can see in the video, holding the '*' key and entering the code causes the indicator light to turn on for ~1s as designed, but the relay does not flop.  The voltage meter shows <4V coming out of the 555--not enough for the relay. If I hit the relay with a jumper wire connected to the 5V rail, the relay flops and there is continuity until I press the '#' key or hit the lower coil with 5V,

Testing various locations with my meter, I see that's not the problem.  Here's the video.

The 555 has 4.86V going in on pin 8 and only ~3.7 going out on pin 3.  That's the problem, and I don't know what to do about it. I fussed with capacitor and resistor values with no change.  For the video, I wound up with ad 1000uf capacitor and a 10K resistor, so pin 3 would have output for longer and I could see it on the meter. I tried various combinations of 2.2, 10, 100, and 1000uf capacitors and 1K, 10K, and 100K resistors--the output stayed under 4V.

Researching, I found this web page, which says that the output on pin 3 will be ~1.7V less than the input on pin 8. Charles' Encyclopedia of Electronic Components Volume 2 says the same thing. I'm going to move on.