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Zero-power PIC self-switch!
A very simple PIC self-switching system, that uses zero power when not switched on.
Roman Black - January 2002

How it works

The SCR (Q1) is the heart of the circuit;
  • The SCR remains switched OFF and ZERO current is drawn from the battery
  • When the button (S1) is pressed the SCR turns on
  • The PIC powers up, and operates a LED etc to draw >5mA total
  • With current over 5mA, the SCR stays on even after the button is released
  • The PIC performs its task
  • The PIC then turns off the LED, goes into sleep mode
  • Current is now <5mA so the SCR turns off
  • Circuit again draws ZERO current is drawn from the battery

    Thyristor theory

    SCR's and triacs are thyristors.

    They are one of the early forms of semiconductor invented just after diodes, they are in fact controlled diodes. SCR means Silicon Controlled Rectifier.

    Apart from their excellent ruggedness, the most important feature of thyristors is that once they are turned on they STAY on!

    Mostly now they are used for phase-angle mains power controlling like drill speed controllers and electric heater controllers. In the earlier days SCR's were used for more interesting tasks like special purpose oscillators and even as the switching components in TV sets and very early switchmode PSU's.

    The entire operation of the circuit depends on the fact that the SCR will remain on when the current is >5mA. The PIC makes this happen. When the PIC later reduces current to <5mA (the SCR Ihold or Ih value) the SCR turns off.

    The circuit could also be used without a PIC, with something like a 555 timer driving a buzzer or waterpump etc so after activation the task is performed for a set time and then when the 555 turns the load off the SCR also turns off and the whole system returns to a zero current state. Useful for an energy efficient pump floatlevel or similar.

    Tips for using it

    SCR turn-on requirements. Most SCRs this size and price range require Igate >0.2mA and Vgt >0.8v to turn on and conduct. This is very easy, the resistor values shown will work with most batteries.

    SCR hold requirements. Most SCRs need Ih >5mA to remain in hold (on). In the first few instructions in the PIC you should set at least one pin as an output, and either activate "the load" which must be more than 5mA, or my preference to drive a led at >5mA from a PIC pin through led and resistor to ground. That will hold the SCR on until you are done. You must keep the PIC circuit drawing >5mA at all times. Just putting a 470 ohm resistor from one PIC output hi pin to gnd will also do it with no led.

    SCR turn-off requirements. Make sure after the PIC has done it's task that you turn all outputs to off, which should be their natural high impedance state. Then put the PIC into sleep and when the total SCR current falls somewhere less than 5mA the entire device will turn off.

    Watch out for some 7805 regulators that can have large quiescent currents, maybe over 5mA used by the 7805 even with no load. Test the 7805 quiescent current by attaching Vin and gnd but not Vout, and use your multimeter on 20mA range.

    It is possible that spikes etc may cause turn-on of the SCR by causing a brief positive voltage at its gate, or that negative current spikes through the PIC (or it's load) may drive the total current below the hold current for a short period of time causing premature turn-off. Adding a small cap (0.1uF) across R2 will help keep SCR turned off if it is turning on by itself. Reducing the value of R2 may also help.

    If you get problems with premature turn-off, try increasing C3 to 47uF or 100uF and maybe add a 0.1uF cap in parallel with it.

    - end -

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