THE ELECTRIC WAVE next page

POWER FLASHER

There is no need to resort to complex circuitry if what you are looking for is a simple power flasher. The light will flash at around 1Hz with a 100W bulb at a duty cycle of 50%. The max. load that can be driven with this circuit is 200W and if you wish to have a different frequency you have to change the value of the capacitor. Operation at 110VAC has not been tested although I expect it to work provided the resistors are set at about half the stated value. The SCR is manufactured by Siemens but any other equivalent semiconductor should work fine. WARNING! - This circuit is directly connected to the mains and proper safety precautions should be taken.

SCR OSCILLATOR

Silicon controlled rectifiers (SCR) can easily oscillate if there is an inductor (a speaker coil in this case) which gives just enough extra voltage to completely switch off the sustain current. In this way a new cycle may start and oscillations set in. It operates over a wide range of supply voltage and components values are not critical at all. Operational frequency in this circuit goes from 100Hz at 11V to 10KHz at 100V.

ZENER OSCILLATORS

These two circuits are interesting from an academic point of view. Their practical implementation is rather critical and it is not easy to get steady operation. Circuit (a) requires a "cooked" zener: connect it first to a constant current generator, then increase the current until the voltage across the zener starts to decrease. Reduce the supply current and wait a few minutes until it really warms up. The zener is now ready for the circuit: increase the voltage slowly until it oscillates (1KHz in the circuit shown). You may need to decrease the voltage once oscillation takes place. With suitable circuit components it will oscillate up to 20MHz. Circuit (b) will oscillate at a very low frequency, normally 2-5Hz, provided the voltage is increased very slowly, loading is critical and you may find that a slightly different lamp will work better. Higher voltage zeners work better than low voltage zeners and the circuits operate only with the specified types. The reasons for the oscillations are unknown, although, for circuit (b) it is felt that some kind of reversible thermal breakdown is at work.

220V MAINS MONITOR

With this circuit you will be able to monitor the quality of the mains. There are 4 distinct sections, each supervising a parameter pertinent to the quality of the supply line. The noise section consists of a 50Hz filter and a speaker where you will hear the noise present on the line. The bicolor LED should be adjusted for the least light with the 5k pot and gives a visual indication of noise or asymmetry in the wave. The second section will detect any spike which is overimposed on the mains voltage: adjust the pot so that it will not trip if you just switch on the light, sensitivity is high enough to detect a switching operation from a close neighbor. The buzzer will beep for about 1sec anytime there is a spike. The actual voltage is detected with section 3: the yellow led will blink at a rate of 6 Hz but will visibly double to 12 Hz for a 10% increase or will come to a halt for a 10% decrease of the voltage. The last section will show the flutter or slow variations of the mains voltage. The circuit will work for a 220V mains: for 230V operation, change the 27V zener to 39V and for 60Hz operation change the 3.9k resistor to 3.3k and the 47k resistor to 39k. Operation at 110V will call for a major redesign of the component values and has not been attempted.

ULTRA LOW FREQUENCY RECEIVER

The frequency covered is from 0.1Hz to 10Hz and useful signals are received up to 16Hz. The first Op-Amp, properly shielded, must be installed close to the antenna (1-3m long) and connected to the rest of the circuit with a 5-core shielded cable. Adjust the 100k trimmer so that the DC setting at the output of the OPA124 does not change when turning the 220k sensitivity pot. A low pass filter followed by a notch filter take care of the mains induced noise. The values in brackets are good for a 60Hz mains. 1% components should be used for the 3 resistors and 3 capacitors of the notch filter. A voltage controlled oscillator gives an audible frequency that follows the input signal and it is very handy if the unit is made portable although I found that just walking around is enough to bury the signal being received. The output signal goes first to a meter and then is available for the connection to a data logger, which is an almost essential part of the receiver. Sensitivity is quite adequate: any TV set switching on the area will be detected. There are also a host of other mysterious signals of unknown origin. The input protection diodes are special low leakage type and should not be replaced by standard diodes. These diodes can be dispensed with if the antenna is installed with care and away from strong electric fields. The diodes connected to the meter are Schottky diodes and will provide a bias against very small signals (mostly noise) which will not go through to the data logger. Pin connection for OPA124: 1 and 5: DC set, 2 and 3: inverting and non-inverting, 6: output, 8: substrate. Pin connection for LF412: 2 and 3: inverting and non-inverting, 1: output, 6 and 5: inverting and non-inverting, 7: output.

ELECTRIC FIELD DETECTOR

The circuit will come handy when you have to follow the mains wires buried in the wall or even water pipes provided they are not too far away (2-4cm max). It will also detect a conversation on a telephone cable without actually touching it (for testing purposes only) and it works as a microphone if you keep a plastic kitchen foil between the probe tip and your mouth. The probe is just the 12mm long gate lead of the transistor. The 33M resistor should be cut short and soldered where the lead enters the transistor case. Connection to the other part of the circuit is via a standard coaxial cable of any length. The input is not protected and a pair of low leakage diodes (JPAD5) could be connected back to back between gate and ground. I did not find them necessary: I covered, with a small piece of plastic sponge, the probe tip to avoid direct contact with electrified surfaces and used a minimum of care when handling the probe.

SCR INVERTER

The only drawback with this circuit is that it might latch in the conducting state if the load is too heavy or if there is a short at the output, this requires some kind of protection, on the input line, in the form of a fuse or similar. The transformer used is a 10W mains type with 6V+6V windings on the SCR side and a 110V+110V windings, in series, at the output. Efficiency is 50% and the ideal load is equivalent to a 22k resistor, 5W. The output waveform is vaguely sinusoidal at a frequency of 400Hz.

SOUND TRIGGERED FLASH

If you wish to take a picture of a fleeting event which generates a sound, you can do it with this sound activated trigger. It does not require any power supply: it feeds on the high voltage available on the flash trigger terminal. Any economic ceramic microphone is suitable for the purpose. The 68nF capacitor introduces a small delay in the operation of the flash and may help in getting the picture in exactly the right moment although you should expect to take several shots for best results. With this circuit you will be able to catch a cork leaving the champagne bottle or the moment a balloon is punctured. The whole circuit could be assembled in the mike housing making a very compact device.

Eager for more           Full astern to main page

Contact: