PP help ?

[mykaitch]

I have been tweaking my pp and have a sort of prob. I fitted a 10turn 100R sense pot and agjusted the other values accordingly. This gives me great control - almost 3" for a 5c coin. Prob is temp. I think that it is the transistor driving the led. As it warms up its Vbe changes and I have to re-adjust the sense pot. A scope on the base shows no drift in the 'integrated' pulses. I am thinking of fitting a comparator instead.All thoughts very welcome.
(Circuit in my other pp posts)

[Davor]

I'd have to put this circuit into spice to see all it's features. At first glance... I seem to find D7 a bit lonely and misplaced there. It is not a good practice to set working condition of a transistor with any other kind of diode than a transistor of a same kind connected as a BE diode (collector and base connected together), also with series resistors of same magnitude as those in emitter degeneration: R6, R7, and R10. That will take care of your temperature problems.

Otherwise I like your circuit.

[mykaitch]

I'd have to put this circuit into spice to see all it's features. At first glance... I seem to find D7 a bit lonely and misplaced there. It is not a good practice to set working condition of a transistor with any other kind of diode than a transistor of a same kind connected as a BE diode (collector and base connected together), also with series resistors of same magnitude as those in emitter degeneration: R6, R7, and R10. That will take care of your temperature problems.

Otherwise I like your circuit.

Thanks for that valuable advice. It does work great, at the right temp. I am going to put all your advice to work and stop making the Mk2 for now. I'll let you know what happens. I have got almost 3" with a 5c so if it is stable it is fine for me. The mystery dim led came back after assembly so I will live with that. I did hang a scope lead over the probe when assembled and thee is a very healthy signal, as you would expet!