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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
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
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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