If there is a need to feed very low power devices you may resort to infrared optocouplers, solar cells, batteries or low power transformers although the latter might be rather oversized for the intended purpose. Eventually, with the exception of solar cells, all of them draw power from the mains so it might be convenient to use a piezoelectric transformer if the power required is in the range 0.1 to 0.3 mW. The schematic shows an easy implementation of such a transformer. Two piezoceramic sounders are glued back-to-back so that the mechanical movement of the first, the primary, is transferred to the second, the secondary. The ac output voltage can be used as it is or rectified in order to feed micropower electronic equipment or trickle charge small backup batteries. The actual implementation requires two ceramic sounders with high intrinsic capacitance: sounders with 80 to 110nF are readily available and usually come as 50mm discs. Two of these discs are cut down to 35mm in order to have a more compact unit and a lower stray capacitance between primary and secondary. A layer of double-sided adhesive tape is laid on the larger plate of each sounder in order to assure proper electrical insulation between primary and secondary. The sides of the sounders are then pressed against each other and the transformer is ready to operate.

R  Load



100 KΩ



47 KΩ



22 KΩ



10 KΩ



4.7 KΩ



The table shows the measured output under several loading conditions: the ac output was measured with the load directly across the output terminals while the dc output was measured with a full wave rectifier in place. The measured dc voltage refers to a schottky bridge rectifier but the use of standard 1N4004 diodes will only show a modest 6-8% voltage decrease. Measurements were taken with the transformer operating in free air, without any holder, but a proper mechanical layout would require the transformer to be firmly held by the edge of the disc. The use of a plastic box is mandatory for safety reason and improves the transfer of mechanical energy to the secondary thus obtaining a 15-20% voltage increase.


The principle of operation of a car hooter has been applied to both a ceramic sounder and a loudspeaker. A break in the supply current is caused by the vibration of the ceramic sounder plate or the speaker membrane. You could implement similar circuits even without a transformer but the voltage range will be limited, there will be too much sparking at the contact point and pressure and position of the contact become critical. The transformer introduces a feedback mechanism thus eliminating or drastically reducing all mentioned negative effects. An output transformer is used in both circuits: one of the winding is normally 4 or 8Ω while the other is at a higher impedance. The larger plate of the piezomechanic oscillator goes to positive through the contact, typically an adjustable screw, and the transformer low impedance winding. To get the correct phase relationship you may need to reverse one of the windings. A similar transformer is used for the electromechanic oscillator with the low impedance winding connected to the speaker. Also in this case you may need to reverse one of the windings but first you must make sure that the speaker cone goes forward when the voltage is applied: reverse the speaker connections if necessary. A small copper strip is glued on the back of the speaker membrane with a screw placed in the speaker casing so that it just touches the copper strip.

Frequency of operation is from 1 to 1.5 KHz for both oscillators. The frequency for the electromechanic oscillator depends mainly on the speaker damping factor: best results are obtained with the speaker laid against a flat surface or sealing the front side with a wooden panel. Operation below 0.4-0.6V depends on the careful adjustment of the screw and mechanical precision of the assembly.



The charger is suitable for lead-acid car batteries and it is assembled in two units: a metal box with the toroidal transformer, instrument, lights, etc, and a small plastic box housing the voltage and temperature circuit. Connection between main box and sensor is realized with a standard 3 core x 1mm, electric cable, 4m long. Its resistance is factored in the circuit calculations and it is the limiting resistor against overcurrent. Do not change type or length as it may alter the overall performance and safety of the charger. The sensor box is typically positioned close to the battery to be charged and two short flexible leads, 2mm section, 30cm long, one red and black the other, terminated with good quality clamps make up the connection from the sensor to the battery.

This solution assures that the battery is charged up to the correct voltage which depends, in turn, upon the ambient temperature. The final voltage should be set, with the 200Ω  multiturn pot, at 14.8V at 20C-68F and derated +/-30mV/C (17mV/F) at any other temperature. For example, if the prevailing ambient temperature is 10C then the final voltage should be set at 15.1V, if the prevailing ambient temperature is 30C the final voltage is 14.5V and so on. Once set, the circuit will automatically adjust the voltage to within 1C. You have to connect a battery in order to carry out this setting.


A thermistor would have simplified the circuit but its correct implementation is not easy and it was preferred to employ a number of diodes. A red led in the sensor box gives an indication of the correct connection to the battery. However the circuit is quite tolerant to mistakes: shorting the output will do no arm as there is no voltage at the output terminals, not until you connect it to the battery. It is the battery voltage that triggers the circuit into operation and once it is disconnected from the battery the voltage too disappears from the output. Only if the battery voltage is above 7-8V then the circuit will operate. A reverse connection of any battery will do no arm either as the circuit will simply not operate. It will withstand a temporary connection of a 24V battery; above this voltage the input circuit is overloaded and could be damaged.

Current control is achieved by switching the SCRs at the appropriate time through the BF761 collector current. The blue led, but any other colour will do, gives an indication that the unit is charging the battery. The led will start flickering at the end of the charging cycle so you know at a glance that the charge is coming to an end. You may leave the battery connected after it has fully charged as there will be a trickle charge which will keep the voltage at its optimum level. Switching noise is eliminated by the 85μH choke made up by winding 27 turns of 1mm enamelled wire on a ferrite ring 27x11mm. Due to the way SCRs operate, the common line is positive and not negative as one would expect. Care must be exercised when connecting all the polarity sensitive devices.

A toroidal transformer has many advantages: it is small, highly efficient, will tolerate a moderate overload and will consume little power, only 3.5VA when switched on and no battery connected. Cost, at this power range, is surprisingly close to a traditional transformer, yet, the inrush current when switched on can be so high, depending on the exact time with respect to the mains sinewave, that the collapse of the ensuing strong magnetic field will produce mighty spikes up to 500V at the secondary, destroying whatever they find in their path. A few capacitors, the use of fast diodes UF4006 and the high voltage transistor BF761 take care of the problem. The main switch should be rated at 10A.

SCRs can get rather hot; the best solution is to mount them on the metal case itself using appropriate insulating kits. As a consequence the box will warm up especially at the beginning of the charging cycle when the unit may be temporarily overloaded. A thermal switch is provided to cut out the mains supply under extreme temperature and overload conditions. This switch is mounted at about 6-8cm away from the SCRs so that it will take care of the heat coming from other sources as well, such as the transformer and the choke.

The unit has been tested with batteries from 44 to 100Ah for over a year, from 0 to 38C (32 to 100F); the upper temperature limit caused the thermal switch to operate. I should relocate the thermal switch in a cooler place if the designed max operating temperature of 40C-104F is to be met. You may have different temperature limits depending on the mechanical configuration of the box and internal components layout. Pay attention to the fact that this charger behaves like a fast charger for the smaller batteries and precautions should be taken concerning gas production and it is good practice to disconnect the battery from the car before charging it.




This power audio oscillator could be used as a warning signal for alarm systems or to attract attention if something is wrong with an equipment. The oscillator, about 750Hz, exploits the characteristic of certain NPN transistors, in this case a BC337, to oscillate if reverse biased and with the base open. Other equivalent transistors might not work. Despite its simplicity the circuit is quite flexible: the 390Ω resistor, normally connected to negative could be switched in through a logic circuit, so it can be driven directly by the circuit to be monitored; the base is normally not used but frequency modulation of the circuit is possible by connecting a modulating signal to the base via a high value resistor, typically 2.2MΩ.

A 3W loudspeaker is adequate for the circuit and it can be either an 8 or 4Ω loudspeaker, although in the latter case a small heatsink is necessary for the BD436. Peak current for an 8Ω loudspeaker can be as high as 1.2A but because the duty cycle is relatively small, the average current was measured at 0.2A hence the overall power requirement is only 2.4W despite the high volume the circuit is capable of. The feed line must be well filtered and can be anything between 9 and 15V although adjustment of the resistor might be required as the oscillation frequency is sensitive to the supply voltage.






Since a new electricity meter, the electronic variety, was installed at my place, I get cut off if I exceed the set power level, 3.3KWh in my case.

The new meter is unforgiving and although there is a little tolerance built in, you really never know when it has gone over the cut off limit, given the number of electric appliances which are continuously switched on and off.

The circuit was designed to give an audible warning when the 3.3KWh limit is exceeded. The transformer is a disused transformer from a soldering gun. It is relatively easy to remove the few turns of the secondary winding and rewind two turns of thick wire, as thick as the wire coming from the meter at least. One turn should be enough if you have a limit of 6.6KWh, but operation at this power level was not tested. As an alternative you may try a small toroidal mains transformer: it is easy to add a few turns of thick wire. Ignore all other windings, if any, except the primary winding, which in our circuit becomes the secondary winding. The circuit is to be installed between the electricity meter with its breaker and the house wiring. With the given components, the circuit will oscillate at 1 sec. on and 1 sec. off, depending on the load. Adjust the potentiometer so that there is no sound below the power limit. The varistor is necessary in case there is a short in the house wiring: the extra voltage at the secondary may damage the circuit. The piezo buzzer can even be placed away from the circuit in any place where it can be easily heard.

It goes without saying that you must know what you are doing as working with the household mains can be dangerous and remember to switch off the mains breaker before doing any work on the electric wiring. Do not attempt to install this circuit if you have doubts on its operation, connections and relevant safety measures.




Occasionally you might have a need to keep a light on for a certain time, usually a few minutes, and be sure that it switches off even if you forget to turn off the switch. This could be useful in a cellar or in a closet. The circuit will switch on a light bulb simply by pressing the push button. After a time of 3-7 minutes it will switch off automatically. The long delay is achieved by partly using the leakage current between anode and gate of the scr. This current is dependent almost on anything: voltage, temperature, lamp power, scr device, etc., this is the reason why the timing is not constant but for the intended application it is not important. If the delay is too short you may increase the 220nF capacitor up to 470nF. Too high a value will keep the light always on. It will work with incandescent light bulbs only. its operation with electronic lamps is erratic and the delay is only 1 or 2 minutes. The scr must be the sensitive gate type and no other type was tested except the TIC106N.

The circuit is rather small and could be housed in the same case as the push button, if there is enough room. Of course you have to substitute the standard switch with a push button. Operation with a 110Vac mains has not been tested although I expect that a 100μF 100V capacitor instead of the 47μF capacitor should do the trick.

It goes without saying that you must know what you are doing as working with the household mains can be dangerous and remember to switch off the mains breaker before doing any work on the electric wiring. Do not attempt to install this circuit if you have doubts on its operation, connections and relevant safety measures.




SCR AUDIO OSCILLATORThese circuits follow a similar one shown in a previous page but operate at 5V, and down to 2V (1.5V for the circuit with the transformer), and it is also louder. To set them properly start with the trimmer at its highest resistance, connect the 5V supply and slowly adjust the trimmer until oscillation sets in. Oscillation frequency can be adjusted, within limits, using the same trimmer. The loudspeaker must not have a DC resistance bigger than 5.5 ohm or else the higher damping factor will prevent the scr from switching off and it will latch on forever. This means that you cannot use a speaker too small, less than 2 inch diameter. For the same reason it is proper not to use any electrolytic capacitor. If, while setting it up, you find that the scr has latched in the on state, temporarely remove the supply voltage or temporarely make a short between anode and catode.  In order to operate it at 12V you must change the resistor from 82 to 120 ohm and the trimmer from 1k to 2.2k with an additional 1Kohm resistor in series. The circuit on the right uses an output transformer, admittedly not readily available nowadays but was common on old transistor radios and amplifiers some years ago. Operation and setting is similar but you may need to invert the secondary winding in order to have a better stability corresponding to a lower oscillation frequency. In this last circuit it might be possible to use a low esr electrolytic capacitor instead of the standard capacitor. If the voltage is increased beyond 5V pay attention to the maximum current flowing through the components, would the scr latch on. Other scr types should work well even if you might be required to adjust the trimmer value.




circuit 1Here we offer some circuit configurations for night lights, other similar schematics are easily available from the web.  In these circuits we have used a plastic case from some old night lights: the neon bulb and relative resistor have been removed to make room for the circuits shown on the left. The first circuit has 4 leds of any color, although the white ones seem to be the best option. Power consumption is about 270mW. Please pay close attention to the leds connection: if you make a mistake in connecting just one of them, then all of them will blow up on the spot. The only critical component is the line capacitor, 220nF, which must be properly rated for the mains voltage, at least 400V. The resistor in parallel with the capacitor has the purpose to discharge it in a relatively short circuit 2time to avoid any electric shock if the plug pins are touched soon after it is removed from the relevant socket. This resistor is not required if the circuit is permanently wired to the mains and there is no chance to touch the connecting pins. The 47nF capacitor, together with the 560 ohm resistor is used to protect the leds against voltage spikes due to lightning, switching operations of highly inductive loads on the same supply line or simply due to the connection of the night light in the relevant socket.

All resistors are 1/4 or 1/2 W and the voltage rating for the 47nF capacitor is 50-100V. Perhaps the only drawback of this circuit is that there is too much light emanating from it; During circuit 3the night our eyes are accustomed to darkness and this circuit might be too bright for our taste and it is therefore most suited to light up a large area rather than a single room. 5mm, clear type leds were used in all these circuit.

The second circuit is a less powerful version with two leds only and really meager power consumption, just 11mW. This version is the one which more closely matches the concept of night light: low power, not a blinding light but enough for our purpose and a limited cost to build.

The third circuit is a combination of the others and it is housed in a case that comes complete with a switch; in fact, some commercially available night lights have a switch to activate or deactivate them and here we use the switch to choose between two power levels: a weak one and a more powerful one.

Power consumption is 15mW with the low circuit 4power setting, open switch, and 310mW with the high power setting, closed switch. The power rating of commercially available night lights is between 0.6 and 1W while the old fashioned type with the little neon bulb is between 0.21 and 0.33W.
In this third circuit we used colored leds: the resulting effect is pleasant because the room is bathed in a light close to a white light but the source is colored.

In the fourth circuit, a very small 80mAh NiMh battery was added. Several tests were carried out to find the best matching among the charging current, led color, number of battery elements, two in our circuit, and the time the green led is shinining when there is no mains. Normally, closed switch, the NiMH battery is charging while the green led will avoid any overcharging effect. The switch comes handy when you do not use the night light thus avoiding the battery discharge. If you wish, the green led can be exchanged with the blue led or you may use white leds for all three of them with the advantage of a longer illumination time when there is no mains. A very nice color combination is to replace the red led with a yellow one and the green led with a white one. Only three leds were used in this circuit. This was necessary to reduce the excessive reverse voltage across the red led. Power consumption is around 140mW. Please note that the forward voltage of the green led will ultimately set the battery voltage. This means that other green leds might give a slight different battery voltage. The one used in this circuit would set the battery voltage at 2.78V. This voltage becomes 2.8V with a blue led and 2.82V when white leds are used. With no mains, we have a range of three days for the circuit with the white led connected to the battery and about two days with the green led and one day with the blue led. The light will go on for twice or three times longer with ever decreasing output.The light intensity is minimal but it is sufficient in complete darkness. Remember that NiMh batteries have to go through 3 or 4 charge-discharge cycles in order to get the best efficiency.
The last circuit is a classic diagram with bridge rectifier
(800V, 1A) and a higher capacity battery. Eventually this is the circuit I adopted for my purposes. I liked the colour combination with the blue light when there is no mains.
The circuits operate from a 230Vac mains but they can be scaled to work from a 110-115Vac mains by doubling the capacitors value and halving the resistors value and voltage rating.

It goes without saying that you must know what you are doing as working with the household mains can be dangerous. Do not attempt to build this circuit if you have doubts on its operation, connections and relevant safety measures.


led light from solar cellThe solar cell is always feeding the Lithium rechargeable battery and if the voltage goes over a given threshold, about 3.8V, then the white leds will switch on. The same leds, 4 of them, are used to protect the battery from overcharging. All leds will switch off when the battery voltage goes below 2.8V and they will switch on again when the battery voltage is again 3.8V. The on and off voltages become 3.9 e 2.9V respectively at freezing temperatures. The circuit was designed for the highest efficiency with a current drain of less than 1A, with leds off.
Instead of lithium accumulator, you can try NiMh batteries, three cells in series will give the right voltage and will give more flexibility: if, for example, the circuit has one only white led, then you need a smaller 600-700 mAh NiMh battery. Also the solar cell must be designed to give a maximum output of 25mA. The drawback of using NiMh batteries is that they do not work very well at freezing temperatures.

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