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The following diagram depicts a circuit intended for activation via reed switches. Q1 is normally conducting through the voltage divider R1 and R2. With Q1 conducting, Q2 is biased off (non-conducting) via R3 and R4. The SCR remains non-conducting. When the magnetic reed switch opens, Q1 no longer conducts, after which C1 discharges into the base of Q2 via R3, which causes Q2 to conduct. The SCR receives current through C1,R5 and R6 and it activates the relay. I'd like to know whether this circuit benefits from filtering components, considering the reed switches (window contacts) enter image description hereare connected via long wires to the circuit and might need components to remove RF and other sources that might induce noise into the circuit.

I was thinking of adding a reverse diode (or TVS) before the first voltage divider. Also, adding 100nF cap at base of Q1. Please share your suggestions.

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  • \$\begingroup\$ You sure think up some odd circuits.. The cap on Q1 is a good idea, so is the TVS. What everything between the collector of Q1 and R6 is for seems overkill. I'd just tie the collector to the gate of that SCR and tie the left end of R6 to Vcc which I assume is also 9V. \$\endgroup\$
    – Trevor_G
    Mar 25, 2017 at 20:11
  • \$\begingroup\$ Yu also need a way to turn it off/bypass it. \$\endgroup\$
    – Trevor_G
    Mar 25, 2017 at 20:13
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    \$\begingroup\$ Hi Trevor, thanks for the suggestion. If I connect the left end of R6 to VCC, wouldn't the SCR always be on? \$\endgroup\$
    – Baphomet
    Mar 25, 2017 at 20:18
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    \$\begingroup\$ It does seem like an unusually complicated way of doing it. In good circuit design, every single part must be justified or not be there. I would have your 9 V signal driving an RC low-pass filter (series R into C to GND) to remove transients/noise then into your SCR. I would want to have to pull a decent current through the switch for a trigger to happen, at least 1 mA, so that radiated noise couldn't do it. I imagine the SCR will require more than that, anyway. \$\endgroup\$
    – TonyM
    Mar 25, 2017 at 20:28
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    \$\begingroup\$ NO point putting the cap before the divider since the R component of RC would be close to zero. On the base of Q1 is the spot. \$\endgroup\$
    – Trevor_G
    Mar 25, 2017 at 20:30

1 Answer 1

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BTW: you can make a simple alarm with just the right relay and a couple of switches. When armed the relay is pulled in.

Not really battery friendly though since the coil is draining current all the time.

schematic

simulate this circuit – Schematic created using CircuitLab

Or a fancier version....

schematic

simulate this circuit

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    \$\begingroup\$ It's simple and efficient design. It would be good for the mains powered circuits. \$\endgroup\$
    – Baphomet
    Mar 25, 2017 at 20:56
  • \$\begingroup\$ Is it self latching? \$\endgroup\$
    – Baphomet
    Mar 25, 2017 at 21:08
  • \$\begingroup\$ yes, since the relay itself is in the switch loop. \$\endgroup\$
    – Trevor_G
    Mar 25, 2017 at 21:10

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