Thoughts on input protection for this BJT based circuit

Thread Starter


Joined Jan 31, 2017
Could you please spare a moment and have a look at this circuit. Once Q1 is on, Q2 is biased off and it must stay off as long as Q1 is on. When reed sensor opens, Q1 turns off and a short pulse from C1, through Q2, turn the SCR on. The SCR is a sensitive type. This activates the relay. My question is, due to reed sensors being far and therefore long wire for connection, obviously there will be RFI/EMI issue, so what do you think about input protection C2,C4 and C5, arranged as low pass filter? please note R1 and R3 must be high value to minimise the current consumption from the battery.
Kind regards



Joined Jun 22, 2012
D2 is useless, it will increase the gate voltage, i would put a 100uF cap from D1 Cathode to battery negative, this will hold the supply for the thyristor.


Joined Dec 29, 2008
Is there an obvious problem when the switching occurs?
The long input wire may induce some type of anomaly ... spreading out of corner waveforms ... something like that.