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.
Pump controller for solar hot water system
This circuit optimises the operation of a solar hot water
system. When the water in the solar collector is hotter than the storage tank,
the pump runs.
The circuit comprises two LM335Z temperature sensors, a
comparator and Mosfet. Sensor 1 connects to the solar collector panel while
Sensor 2 connects to the hot water panel. Each sensor includes a trimpot to
allow adjustment of the output level. In practice, VR1 and VR2 are adjusted so
that both Sensor 1 and Sensor 2 have the same output voltage when they are at
the same temperature.
The Sensor outputs are monitored using comparator IC1. When
Sensor 1 produces a higher voltage than Sensor 2, which means that sensor 1 is
at a higher temperature, pin 1 of IC1 goes high and drives the gate of Mosfet
Q1. This in turn drives the pump motor.
IC1 includes hysteresis so that the output does not oscillate
when both sensors are producing a similar voltage. Hysteresis comprises the
resistor between output pin 1 and non-inverting input pin 3 and the input
This provides a nominal 12mV hysteresis so that voltage at Sensor 1 or Sensor 2
must differ by 12mV for changes in the comparator output to occur. Since the
outputs of Sensor 1 and Sensor 2 change by about 10mV/°C, we could say that
there is a degree of hysteresis in the comparator.
Note that IC1 is a dual comparator with the second unit unused.
Its inputs are tied to ground and pin 2 of IC1 respectively. This sets the pin 7
output high. Since the output is an open collector, it will be at a high
Mosfet Q1 is rated at 60A and 60V and is suitable for driving
inductive loads due to its avalanche suppression capability. This clamps any
inductively induced voltages exceeding the voltage rating of the Mosfet.
The sensors are adjusted initially with both measuring the same
temperature. This can be done at room temperature; adjust the trimpots so that
the voltage between ground and the positive terminal reads the same for both
sensors. If you wish, the sensors can be set to 10mV/°C change with the output
referred to the Kelvin scale which is 273K at 0°C. So at 25°C, the sensor output
should be set to (273 + 25 = 298) x 10mV or 2.98V.
Note that the sensors will produce incorrect outputs if their
leads are exposed to moisture and they should be protected with some neutral
silicone sealant. The sensors can be mounted by clamping them directly
to the outside surface of the solar collector and on an uninsulated section of
the storage tank. The thermostat housing is usually a good position on the
Battery equality monitor
Almost all 24V power systems in trucks, 4WDs, RVs, boats, etc,
employ two series-connected 12V lead-acid batteries. The charging system can
only maintain the sum of the individual battery voltages. If one battery is
failing, this circuit will light a LED. Hence impending battery problems can be
forecast. The circuit works by detecting a voltage difference between the two
series connected 12V batteries. Idle current is low enough to allow the unit to
be permanently left across the batteries.
G. La Rooy, Christchurch,
New Zealand. ($30)
Component & voltage tester
This simple circuit tests speakers, microphones, transformers
and voltage. It's basically a very low frequency oscillator that produces
extremely short 'fruity' pulses. The type of sound produced is very easy to hear
and to determine the precise direction it is coming from, thus making it ideal
for checking the phasing in multiple speaker installations. It is also very
useful for car stereo installations as well as public address systems where it
can drive dozens of speakers directly on a 100V or 70V line system.
The signal is also easy to hear on a public address system so
that you can drive around a large installation with the window down and easily
hear each speaker as you drive past. It is easy to check that a speaker is in
phase with its neighbours, by listening for the artificial centre created
between two identical sound sources.
Q1 and Q2 oscillate when connected to loads between zero and
about 1000Ω. The
frequency increases as the resistance of the load increases - 8Ω loads produce about 8Hz
output while 100Ω loads will produce about 100Hz output, although it is only
The unit is also useful for checking dynamic microphones (not
condenser types), headphones, transformers (both audio and mains) and resistance
loads (only visual checks via the LED). The pulses produced can sound too loud
for some delicate circuits such as dynamic microphones and headphones, but the
pulse is so short that it is virtually impossible to do any damage; the average
current flow is only a few milliamps.
The circuit needs no power switch as the oscillator only
operates when the negative side of the battery is connected through the load
being tested. The LED flashes at each pulse as a visual indication that the load
is lower than about 1000Ω. The circuit works from a 3V battery pack. To use a 9V
battery change the 15Ω resistor to 47Ω, the 1.8Ω resistor to 5.6Ω and the .033μF capacitor to .01μF.
LED2, diode D1, zener diode ZD1 and the series 220Ω resistor form a voltage
indicator which is used to detect and indicate any voltage greater than about
10V. LED2 only illuminates if the voltage rises above the threshold set by ZD1
and D1, which is more than the battery voltage (3V or 9V). These components can
be omitted if the device is not going to be used for working on cars. However,
it's quite handy having a device that can check power wires, shorts to chassis
and speakers in a car.
Launceston, Tas. ($30)