Switch circuit problems.

Hello,

I have an IR switch circuit composed of an IR LED, IR phototransistor
to LM393 comparator with feedback to a 74HC74 positive edge flip-flop.
The F/F had contolled a small relay via an NPN transistor and worked
fine. When I decided to replace the relay with an NPN power
transistor or NPN mosfet I found it wouldn't work correctly.

If they switched appox 50mA then it would turn on and off but if they
switched (350mA) it would turn on and stay on. I tried various
different transistors and base resistor values to no avail. The mosfet
I connected directly to the F/F output and then tried switching it
with a small NPN transistor without success. However when I tried
adding a resistor (over 10K) between the mosfet and the F/F output it
worked. This I don't understand as I thought the mosfet was a voltage
controlled device and as the F/F output swings nicely from almost 5VDC
V+ to almost V- it stumps me.

Does anyone have an explanation as to why this is?

In addition I have another little IR phototransistor switch circuit
with an NPN darlington pair switching a relay from their common
collectors to V+ upon interruption of the IR beam. How can I
incorporate the darlington pairs amplification into the first circuit
for increased range? Each time I've tried this I've failed to get it
to work.


Cheers,

Andrew.

 

What you should look at is the output of the F/F (when there is no
resistor between the output of the F/F and the mosfet), to see if it is
switching. Flip-flops can be fussy sometimes about the load they are driving
(sometines it will keep them from switching). The resistor between the
output of the F/F and mosfet, may give it the decoupling that it needs.
Also, be sure to have a decoupling capacitor across the power suppy leads of
the F/F.
Brian

 

semiconductors do not have.
this is very common in control circuits. This gives a dead band and not
simply on-off.

Ray

 

Don't forget an inverse parallel diode across the relay coil.