Im having trouble with an SPDT reed realy i'm trying to incorporate into a circuit using a flashing LED to operate the coil, however the output circuit seems to be constantly on..any ideas as to why? im at a dead loss here(coil circuit is 9v dc other circuit is 12v dc)

 

Hi Alex,

I have no idea what SPDT is,
however i assume the reedswitch
is driven like this.

Sometimes the base of the transistor has
to be very low to turn it off,
especially if its a darlington pair.

If this is a mass produced item,
or sold as a working model, then this is
a failure rather than a design problem.

I assume this is a home made assembly ?

One way to get around the high base sensitivity
is to alter the circuit slightly to an
emitter-follower type.

The gain of the stage is still preserved,
but now the low voltage on the base
would comfortably switch off the reedswitch.

This may not be your problem,
but it might be.

Regards, John

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nkyou for you help John, However i have solved the problem by changing relays and stepping the voltage up to 12 volts on both circuits which has given me the desired output pulsing that i required. (SPDT - single pole double throw by the way)

Alex

 

If you think this might be the problem,
just put the reedswitch coil in the emitter instead.
No other changes.

Regards, John

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Glad you got it sorted!

John