Motorized gadget: step #3

OK, so I got trough the previous problems.

       First of all I applied heat sink to the transistor to cool it down. It's much better now, but still the heat sink is a bit warmer than it should be. Maybe some little fan will be attached. Since the heat sink doesn't get hot all the time I will proceed observing this process. For transistor cooling I used old heat sink from CPU and also applied special paste that would better distribute heat between transistor and heat sink. 

    About the second problem (described in last blog post) - I borrowed oscilloscope to see what is happening with the pulse signal that should control the servo.

    To see how the circuit works at first I didn't add the control current - power supply that produces current from 0-10V.

These are the results:

1. Potentiometer in the center position (on NE555 timer pin #5 current is 1.6V):

As you can see pulse signal is normal, nothing unusual.

2. Results when potentiometer is turned left:

We can see that pulse signal is getting longer which is a good sign.

3. Results when potentiometer is turned right:

Pulse signal is getting shorter so again - good sign. 

This let me to conclude that circuit is working fine. 

After this I added the control current (power supply) and results disappointed me.

4. When power supply current is 0 V / it is switched off the results are:

As you can see - no signal.

5. Now turning the power supply on / approx. 7-10 V:

As you can see, the signal is stronger, but it doesn't create pulses.

      So clearly - problem is somewhere in the power supply. Since the power supply I have is for laboratories it is very precise. Therefore - there shouldn't be problems with the power supply it self. 

    I started to look at the description for the circuit at http://www.epanorama.net/circuits/servo10v.html
and encountered that problem was very trivial - control input impedance for this circuit must be greater than 30 kOhms. Measuring impedance on power supply got me precise 20 kOhms.

       So after applying 15 kOhm resistor between power supply and circuit everything started to work.

The result looks like this: 

        Now I have to integrate these circuits in the center console in my car. Soon new posts will follow. 

       Also I have to find solution how to turn on these circuits when I unlock my car and turn them off when I lock my car. I guess I will have to use some relays and car alarm schematics.