Suitable for battery-operated devices, Fixed 35 minutes delay
This timer was designed mainly to switch off a portable radio after
some time: in this way, one can fall asleep on the sand or on a hammock,
resting assured that the receiver will switch off automatically after
some time, saving battery costs.
R1 and C1 provide a very long time constant. When P2 is momentarily
closed, C1 discharges and the near zero voltage at its positive lead is
applied to the high impedance inputs of the four gates of IC1 wired in
parallel. The four paralleled gate outputs of the IC go therefore to the
high state and the battery voltage is available at Q1 Emitter.
When P2 is released, C1 starts charging slowly through R1 and when the
voltage at its positive lead has reached about half the battery voltage,
the IC gate outputs fall to zero, stopping Q1.
This transistor can directly drive a portable radio receiver or
different devices drawing a current up to about 250mA. Connecting a
Relay across the Emitter of Q1 and negative ground, devices requiring
much higher voltage and current operation can be driven through its
Pushing on P2 for 1 to 5 seconds, the circuit starts and then will
switch off after about 35 minutes. This time delay can be varied by
changing R1 and/or C1 values. P1 will stop the timer if required.
LED D1 is optional and can be useful to signal relay operation when the load is placed far from the timer.
R1___________10M 1/4W Resistor
R2___________4K7 1/4W Resistor
R3___________1K 1/4W Resistor (Optional, see Text)
C1___________220µF 25V Electrolytic capacitor
D1___________LED any type and color (Optional, see Text)
D2___________1N4148 75V 150mA Diode (Optional, see Text)
IC1___________4011 Quad 2 Input NAND Gate CMos IC (See Notes)
Q1___________BC337 45V 800mA NPN Transistor
RL1___________Relay with SPDT 2A @ 230V switch (Optional, see Text)
Coil Voltage 12V – Coil resistance 200-300 Ohm
- A 4011 Quad 2 Input NAND Gate was used
for IC1, but many other CMos gates or inverter arrays can be used in its
place, e.g. 4001, 4002, 4025, 4012, 4023, 4049, 4069. With these
devices, all inputs must be tied together and also all outputs, as shown
in the Circuit diagram.
- The operating voltage of this circuit should lie in the 6 – 12V range.