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.

Loudspeaker protector monitors current

This circuit uses a 0.1Ω 1W resistor connected in series with the output of a power amplifier. When the amplifier is delivering 100W into an 8Ω load, the resistor will be dissipating 1.25W. The resulting temperature rise is sensed by a thermistor which is thermally bonded to the resistor.

The thermistor is connected in series with a resistor string which is monitored by the non-inverting (+) inputs of four comparators in an LM339 quad comparator. All of the comparator inverting inputs are connected to an adjustable threshold voltage provided by trimpot VR1.

As the thermistor heats up, its resistance increases, raising the voltage along the resistor ladder. When the voltage on the non-inverting input of each comparator exceeds the voltage at its inverting input, the output switches high and illuminates the relevant LED.

NOR gate latches are connected to the outputs of the third and fourth comparators. When the third comparator switches high, the first latch is set, turning on Q1 and relay 1. This switches in an attenuation network (resistors RA & RB) to reduce the power level.

However, if the power level is still excessive, comparator 4 will switch, setting its latch and turning on Q2 and relay 2. This disconnects the loudspeaker load. The thermistor then needs to cool down before normal operation will be restored.

The values of R1-R4 depend on the thermistor used. For example, if a thermistor with a resistance of 1.5kΩ at 25°C is used, then R1 could be around 1.5kΩ and R2, R3 and R4 would each be 100Ω (depending the temperature coefficient of the thermistor).

The setup procedure involves connecting a sinewave oscillator to the input of the power amplifier and using a dummy load for the output. Set the power level desired and adjust trimpot VR1 to light LED1. Then increase the power to check that the other LEDs light at satisfactory levels.

David Devers,
Kingsbury, Vic.

Modification to Smoke Alarm The Smoke Alarm Monitor described in the January and February 1997 issues does not operate smoke alarms which have test inputs which do not connect to ground; eg, the Family Guard smoke alarm. The solution is to use an optocoupler so that the test input to the smoke alarm can be triggered without reference to the ground terminal. This involves modifications to the original Additional Circuit which needs to be installed into each smoke alarm. The internal LED of the optocoupler is driven via transistor Q4. Originally this transistor shorted the test switch on the smoke alarm. Note that the polarity must be correct, with the collector of the optocoupler (pin 5) going to the more positive terminal of the test switch. This polarity can be determined using a multimeter across the test switch. SILICON CHIP.

Body charge detector

It is well known that through such simple everyday activities as walking on a carpet or moving in a chair, the body accumulates a static charge - sometimes many thousands of volts.

Due to its extreme sensitivity, this circuit will detect not only such charges but also EMF-induced voltages in the body, which are generally far smaller. This means that, whether you happen to be "charged up" on any particular day or not, your body will almost certainly trigger this circuit.

An interesting twist is that the sensor does not need to be made of metal. Provided it is isolated from ground, the sensor can be any conductor, including a plant in a pot.

The circuit is a comparator based on an LF351 FET-input op amp (IC1). The has the benefit of a high impedance input which is crucial for detecting a static charge.

The other aspect which is crucial is that the 0V side of the circuit must be connected to earth (eg, a metal stake driven into the ground). Without the grounded connection, the circuit will yield poor results.

Notice that the sensor connections are taken through diodes D1 and D2. This means that both negative and positive charges will cause the voltage at IC1's inverting input to exceed that of the non-inverting input (the voltage at the inverting input rises or that at the non-inverting input falls).

Trimpot VR1 and the two 470kΩ resistors across the supply are used to set the inverting and non-inverting inputs (pins 2 and 3) at around half-supply (ie, +2.5V), while the two 470kΩ input resistors protect IC1 against damage from static. The sensitivity of the circuit is adjusted by VR1.

While higher static charges will brightly flash the red LED, small and very rapid discharges through the sensor may barely illuminate it. The way around this is to feed the output at pin 6 directly to the trigger input (pin 2) of a standard 555 monostable timer IC. This would then offer a clearer indication of triggering.

This circuit could prove particularly useful as an indicator of static charge before handling sensitive components.

Thomas Scarborough,
Cape Town,
South Africa. ($35)

Preamp stage for ceramic phono cartridge or violin pickups While we have published a number of variations on a standard RIAA preamplifier for magnetic phono cartridges, we have not published a preamp stage for ceramic phono cartridges. Typically, these were supplied as turnover cartridges in record changers but there were higher quality versions such as the Decca Deram. These phono cartridges are piezoelectric devices which require a very high input impedance. Similarly, violin pick-ups made by Fishman, Barcus Berry and others are piezo devices. These two circuits have been requested for a violin pickup but could equally well suit a ceramic or crystal pickup. The op amp circuit uses a TL071 connected as a voltage-follower. It can run from a battery supply of ±9V. The alternative transistor circuit uses a BC549 connected as an emitter-follower but with bootstrapping of the input bias network to provide a high input impedance. Both circuits have input coupling capacitors but since the transducers are capacitive (ie, piezo) they could possibly be omitted. Both circuits will probably need to be followed by further gain, depending on the output level. For a violin pickup, a parametric equaliser is also recommended, and for this we would suggest the 3-band parametric equaliser published in the July 1996 issue of SILICON CHIP. With a slight change to the feedback of the first op amp in this circuit, the extra gain required could also be provided. SILICON CHIP.