Hi MIKE.

14-Mar-04 09:56:40, MIKE ROSS wrote to Jasen Betts

 JB>> the final circuit was something like this.
 MR>  JB>
 JB>>         ___________________________________________________________ +12
 JB>>         |          |      |       |                |           |
 JB>>        ???        10k    _L_     _L_100n   ,-[1M]--)----.      |
 JB>>         |          |     /^\     ~T~       |  R7   |    |  .---|---.
 JB>>         |     _____|  || ~T~ . .  |        |     |/     |  \  _L_  |
 JB>>         |  |/'     |--||--+--|<|--+-[100K]-+-----|~ Q3  `-|/  /^\  RL
 JB>>  ,-[1M]-|--|      /   ||     ` `     R6          |\        \  ~T~  |
 JB>>  |      |  |\| |/'    1n                           |        `--|---'
 JB>> _L_     |   ~`-|  Q1,2                             |          /'
 JB>> ~T~     |      |\|                                 |       |/'
 JB>>  |10p   |       ~`\                                |-[10K]-|   Q4
 JB>>  |      |          |~~~~|                         10K      |\|
 JB>> _|     2M2        3K1  _L_1u                       |        ~`\,
 JB>> video   |          |   ~T~                         |           |
 JB>> input   |          |    |                          |           |
 JB>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 0V
 MR>  JB>

 MR> Thanks for the artwork. I'm assuming you are publishing your circuit
 MR> for all to see in order to get some constructive feedback, so I'll
 MR> try to be helpful.

Thanks.

 MR> I like the addition of the R7 positive feedback. It adds a level
 MR> sensitive trigger point to the circuit with some hysterisis.

That was the idea, an attempt to stop marginal signals from only driving q4
half-way and causing it to heat up.  the relays have inherent in
themselves a measure of mechanical positive feedback.

 MR> OTOH
 MR> might the addition of R7 have possibly made the circuit a little
 MR> susceptible to oscillation because of the coil inductance and stray
 MR> capacitive coupling?

after installing the circuit and the relays together that's exactly what I
found happened, I adjusted the trimpot (perhaps back past the point of
positive feedback) and the oscilation stopped.  if something goes wrong
which cooks Q4 I'll have another look.

another move to stop that could have been to put a shield between the relay
and the electronics. or to re-position the components closer together.

 MR> The original task really boils down to 2 requirements: 1-triggering,
 MR> 2-timing. Your circuit fulfills #1 very well because of the ample
 MR> input gain but I feel it neglects #2 to some extent.

 MR> However you must take my comments with a very large grain of salt
 MR> especially if the circuit in fact does exactly what it should for you!

timing?

 -=> Bye <=-

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