Interesting circuit ideas which we have checked but not built and tested. Contributions from
readers are welcome and will be paid for at standard rates.
Regulator for solar battery charger
Some solar regulators are complicated and tricky to set up, and
may also have relay chatter. This version is easy to set up and works
positively. The relay is a 12V 400-ohm coil type with 10A contacts (such as Dick
Q1 monitors the battery being charged via the 330kΩ and 10kΩ resistors feeding its base.
While the battery voltage is below the threshold set by trimpot VR1, Q1 is off,
Q2 is on and the relay is energised to pass current from the solar panel and
diode D1. Q2 also lights LED1.
When the battery voltage rises above the threshold set by VR1,
Q1 turns on, removing base current from Q2 which then turns off the relay. The
at the base of Q1 prevents relay chatter.
Once the relay is open, a trickle charge can be maintained via
a suitable incandescent lamp which bypasses the relay contacts.
The meters and changeover switch can be regarded as optional.
To set the cutoff voltage (eg, 13.8V), apply 14-15V to the battery terminals and
adjust trimpot VR1 until LED1 goes out at the desired voltage.
Kawhia, New Zealand. ($30)
Simple time switch for battery-operated projects
The use of this simple timer will prevent batteries from going
flat because you forgot to turn off the power. It uses a momentary pushbutton
switch to apply power to the controlled device and then, after a delay of about
eight minutes, power is automatically cut. Timing can be restarted at any time
during the 8-minute interval.
The 4060 14-Stage Ripple Carry Binary Counter governs the ON
timing duration with the R/C combination at its clock inputs, pins 9, 10 and 11,
providing the rate of clock pulses. Q1 switches power in the positive line to
the device to be controlled when IC1's Q14 output goes low. A high-brightness
LED is included as an ON indicator but can be deleted if not required.
IC1 is permanently connected to the battery supply but draws
just a few microamps of current when dormant. This current flows through a
and insufficient voltage is induced across the base-emitter junction of Q1 to
turn it on. The Reset pin of the 4060 is held low by a 47kΩ resistor and as a result,
output Q14 (pin 3) is set high. This also holds pin 11 high via D1, preventing
the clock from pulsing.
When the pushbutton is pressed, two events take place. The
Reset pin of IC1 now becomes momentarily high, resetting all Q outputs Low. With
Q14 low, Q1 then turns on to power the external circuit. After about eight
minutes, the count sets Q14 high again and Q1 turns off.
The timing duration can be varied by using a different
capacitor value on pin 9 of IC1. Supply voltages from 5-15V are acceptable but
the 3.3kΩ resistor should be tailored to provide about 5mA base current to Q1.
Redcliffe, Qld. ($40)
Door monitor for a dog
This circuit was produced to monitor the movement of an old dog
which at times during the night must be let outside. A door monitor set low off
the ground would have sufficed but it needed to discriminate against cats as
The first 555 pulses the infrared LED at 38kHz and the light
output reflects off a car or truck reflector to be picked up by the infrared
detector IC. When it picks up infrared, its output goes low to turn on the
visible LED via the BC558 transistor. This LED serves both as a power on
indicator and is used to set up the reflector.
The height of the transmitter and receiver (optically isolated
but in the same box) was set a little below the shoulder height of the dog. To
prevent false alarms from cats' tails occurring, an RC time delay was included,
comprising the 1MΩ resistor and 47μF capacitor at pin 3 of the LM311 comparator. This comparator goes low to provide a clean trigger signal to the second 555 timer, which
operates as a monostable to drive a flashing LED and/or a piezo buzzer for 10
Electric field proximity switch
This little circuit does not pretend to compete with the Body
Detector sensor featured in the October 2001 issue of SILICON CHIP but its sensitivity for so simple a
circuit is very high. The circuit will respond to both RF and electrostatic
The proximity switch behaves rather like the automatic
door-opener at the entrance to many stores. A person approaching will activate
the device and open the doors but if the person then stops and remains still,
the doors will close.
With no moving objects within its range, the field strength
varies slowly and has no effect on the device. But a large body such as a person
entering its zone of operation results in large changes in the field
Referring to the circuit diagram, the sensor is nothing more
than a stiff piece of wire, insulated or bare, standing vertically like an
antenna, approximately 15cm long. This is connected to the gate of FET Q1 which
is connected as a source follower and operates as a high-impedance buffer with a
low output impedance.
The source of Q1 drives the base of PNP transistor Q2 which
charges the 10μF
capacitor at its collector in response to voltage changes picked up by the
antenna. Q2 also drives Darlington transistor Q3 via a 220kΩ resistor and this, in turn,
drives the relay. The 220kΩ resistor and 10μF capacitor provide a delay so the relay stays on for up to
In use, trimpot VR1 should be adjusted for maximum sensitivity
by setting it at the point where the output just turns off. This is preferably
done before installing the 10μF capacitor.
The circuit may be susceptible to certain nearby electrical
appliances but not normally overhead lighting. In some situations, the unit can
sense the presence of a person from two metres away but its performance depends
on the clothes worn, the level of friction generated, the dryness of the air,
etc. It reacts violently to polythene film.
In its standby state, it draws less than 1mA, depending on the
setting of VR1.
J. A. Lee,
Old Reynella, SA. ($30)
White LED torch circuit uses flash parts
This circuit is essentially the same as in the December 2000
article but has a current regulator to drive the white LED. In effect, the
current regulator senses the LED current and adjusts the pulse width from the
two-transistor multivibrator to vary for the charging of the inductor. The
discharge time for the inductor remains constant.
Hence, with a fresh battery, the multivibrator runs fast and
gradually slows as the battery runs down. This circuit will allow operation down
to a battery voltage of 0.8V with a useful light output. With a battery voltage
of 1.5V the LED current is approximately 20.5mA. A battery of 1.2V, gives a LED
current of approximately 19mA. The efficiency of the circuit varies between 52%
The main switching transistor is a 2SD965. It is available for
free, along with the small inductor (a transformer with a 8.5mm x 8.5mm core)
that is used in this circuit. These parts along with many other useful bits and
pieces come from the electronic flash units that are fitted into cheap
To get them, go to a 1-hour film processing shop and kindly ask
for any disposed cameras from their rubbish. Most places are helpful. Sometimes
you even get the alkaline battery which is still useful. Get several different
cameras because the transistor and tiny inductor come out of different makes of
WARNING: when taking these cameras apart, be aware that the
flash reservoir capacitor may still be charged up to about 300V DC. This can be
LETHAL. Be sure to discharge this capacitor before working on the unit.
The transformer comes apart very easily. Remove it from the
flash unit. Unwrap the tape around the core and heat the core on the barrel of
your soldering iron for a short time. The wax holding the core together will
soften. Using rags to protect your fingers, gently open the core. While the core
is still hot wipe off as much wax as you can.
Be gentle as the ferrite and former are delicate. Remove the
windings from the former. Wind on 24 turns of 0.3mm or 0.4 mm enamelled copper
wire. Wire size is not too critical. Reassemble the core onto the former and
wrap with insulation tape.
When testing, use a 150Ω resistor as the load. The voltage
across the load will be close to 3V. If operating the torch from a 3V battery,
increase the current sense resistor to 27Ω.
Hamilton, NZ. ($40)