This circuit was specifically designed to recharge alkaline cells. The unusual connection of the transistor in each charging unit will cause it to oscillate, on and off, thus transferring the charge accumulated in the capacitor to the cell. The orange LED will blink for around 5 times a second for a 1.37V cell. For a totally discharged cell the blinking is faster but it will decrease until it will come to a stop when the cell is charged. You may leave the cell in the charger as it will trickle charge and keep it at around 1.6V. To set the correct voltage you have to connect a fresh, unused cell and adjust the trimmer until oscillations set in, then go back a little until no oscillation is present and the circuit is ready to operate. You should use only the specified transistors, LED colors, zener voltage and power rating because they will set the final voltage across the cell. A simple 9V charging circuit was also included: it will charge up to around 9.3V and then keep it on a trickle charge: the green LED will be off while charging and will be fully on when the battery is close to its final voltage.

A 2.5VA transformer will easily charge up to 4 cells at the same time although 2 only are shown in the schematic. In order to minimize interference from one circuit to the other they have nothing in common except the transformer and, in order to show a balanced load to the transformer, half of the charging units will use the positive sinewave and the other half the negative sinewave. Make sure to use high beta transistors such as BC337-25 or better BC337-40. Given the dispersion of the transistor parameters it might happen that oscillations do not take place. Use a slightly higher zener voltage: 7.5V instead of 6.8 or a green led in place of the orange ones. The voltage measured at the transformer output should be 9.5Vac for proper operation.

All types of alkaline cells can be recharged: it will take 1 day for a discharged AA cell or 9V battery and up to several days for a large D type cell. The best practice is not to discharge completely the cell or battery but rather to give a short charge every so often although admittedly this is not easy to achieve. Do not attempt to recharge a totally discharged cell or a cell showing even the slightest sign of damage, best recharge is achieved when the battery is still 70% charged. The more the battery is left to discharge the less effective will be its recharge.

I tried successfully to recharge NiMH cells as well. Although the charging profile for these cells is quite different from alkaline cells, the circuit seems to work fine provided you do not leave them in the charger forever, because of the possibility of overcharging especially for the smaller batteries.

The mains transformer must be suited for the voltage available in each country: usually 230Vac or 115Vac.



A single transistor is all you need for this simple inverter. The main aim of this circuit is to provide a suitable supply for all kind of low power battery chargers that normally connect to the mains such as mobile phones, electric shavers, etc, even an electronic neon light rated at 5W was successfully connected. Only easily obtainable components are used. The transformer is a standard 10VA mains transformer with two 6V windings connected as shown in the schematic. Frequency of operation is between 70 and 190Hz depending on the nature of the load. This frequency is acceptable by most devices but obviously it is not suitable to drive frequency dependent appliances such as clocks or small motors that depend on the mains frequency in order to operate reliably. The transistor will not require any additional heatsink if it is assembled on the metallic case provided for the inverter. The neon glow light will give a useful indication, and warning, on the presence of a dangerous voltage at the output. A 2.5A fuse on the input supply line would be a useful addition. Operation is simple: switch on the unit and connect the load keeping an eye on the neon glow light which should be always on: certain switching chargers demand an initial peak current effectively shorting the output and switching off the neon: in this case you have to try repeatedly to connect the load until it works. A temporary short at the output and a temporary voltage reversal at the input will not damage the unit. Efficiency was not a design parameter however it was measured to be between 50 and 60%. If you have a 110V mains transformer and consequently a 110VAC output you should change the 0.1μF capacitor to 0.22μF, 400V. The waveform is only vaguely sinusoidal. Invert the connection of one of the 6V windings if oscillations do not set in.




Checking the status of your car battery (accumulator) should be easier with this circuit which measures the internal resistance of the battery. Pulses generated by the 555 are used to drive a dummy load and the AC voltage which develops across the battery gives an indication of its internal resistance: the lower the voltage the healthier the battery. The AC voltage is read out by means of a digital meter connected at the output. Separate leads are used for the dummy load and for the metering circuit. They should be connected to their respective battery lugs but they should not touch each other. This avoids erroneous readings due to less than perfect contacts of the dummy load. The internal resistance depends on the battery temperature as well; this is the reason for the switch: hot means a battery (not ambient) temperature between 35 and 52 degrees Centigrade, normal is for a temperature between 16 and 34 degrees and cold is good for a temperature from -4 to 15. Beyond these ranges the reading is unreliable. The internal resistance depends also on the rated capacity of the battery. The 100 ohm potentiometer sets the battery capacity: it is rotated totally to positive for a 100Ah battery and totally to negative for a 32Ah battery. A dial with uniform markings from 32 to 100 was used in the prototype. This means we can measure internal resistance of batteries rated from 32 to 100Ah. As there are a number of smaller 12V batteries around, specially for alarm systems, a switch was introduced that, in the X1 position, will change the capacity range to 3.2 - 10Ah. The unit has six leads going out of the box: two for the dummy load, two for the metering section and two going to the digital meter. Operation is simple: set the range, temperature and battery rating, then connect the dummy load and the metering leads to the battery lugs and read the ac voltage: you should be safe if it reads below 10-12mV otherwise it is better to give the battery a good recharge and if it is still beyond 10-12mV then probably you need a new battery. A bright orange LED shows that the unit is connected and in operation.




Full short-circuit and overcurrent protection is given by this circuit suitable for workbench applications in technical schools and laboratories where there is a need to work directly with the mains. Additional features are a clearly visible red lamp indicating that the voltage is present, good isolation of the output circuit when the unit is off, only a few millivolts were measured with no load, current threshold adjustable over a limited range and the possibility of remote cutout: the 6V from the secondary can be taken anywhere, normally where you are working, even far away from the protection circuit. Pressing the push button will short-circuit the winding and the circuit will switch off thus removing the mains voltage. A suitable led is placed together with the push button to show whether the circuit is in operation or not. Additional remote cutout circuits can be wired in parallel if so required. The circuit will switch off if a short is applied at the output without blowing the fuse but it will blow if you try to activate the circuit if a short is already present. If in doubts, first activate the circuit and then apply the load. The BTB12-600SW is a snubberless triac while the T0805 is a standard triac: you may use other equivalent types but because of the way triacs are driven you cannot use, in this circuit, a snubberless triac instead of a standard triac and viceversa. The 250 μH inductor is a coreless inductor made with 100 turns of 1mm enamelled wire over a form 27mm diameter and 12mm wide. The mains transformer is a standard transformer with split primary wired in such a way that the circuit will self-sustain once it is activated. The same circuit was implemented with a current limit between 0.1 and 0.3A. In this case you have to change the fuse from 6.3A to 1.5A and the sensing resistor from 1Ω  to 10Ω. This resistor must be the cemented type not the armoured type. The latter is not able to withstand the temporary high overload that takes place during a short-circuit condition. Voltage drop from input to output is between 1V for little or no load and 3.6V with a load current of 2A.



This circuit will convert a standard relay to a pulse relay; pressing the button will switch it on and pressing it again will switch it off. For this purpose you need a relay with 2 sets of contacts: one is used for the circuit and the other is available for an outside circuit. Sometimes it is difficult or impossible to find a stepping relay, normally used in electrical wiring, and this is a viable solution. The relay used in this circuit was a power relay with 10A contacts and a coil resistance of 28Ω. The circuit will draw no power when idle and it is possible to scale up the circuit to operate at a higher voltage. The relay must be always rated at half the supply voltage, in our case it is a 6V relay for a 12V supply. The resistor in series with the coil must have a similar resistance as the coil or slightly higher and the other resistor should be twice the coil resistance. All capacitors are 25V. The capacitors value depends on the coil resistance: the higher the resistance the lower the value. As it takes a certain time to charge the capacitors it is necessary to wait about 0.5-1sec between one operation of the push button and the next. An unregulated 12V power supply is adequate for this circuit.  



If you wish to have some really nice looking LED's shining out of your equipment panel, you may try the following trick: pass repeatedly a fine sandpaper on the surface of any transparent and clear LED until the same surface is all worked out to a whitish look.


There is nothing else to do but to switch it on and enjoy the pleasant look of it. Do not use the extra fine sandpaper as it will not cut deep enough in the LED plastic material, in other words the sandpaper normally used for metals is not suitable. As the difference with a standard LED was remarkable I did some tests in order to compare


them: picture 2 and 5 refer to the normal clear LED, red in these tests, shining right in front of a screen and tilted at about 60 respectively. The results were as expected: very bright when viewed on axis and dimmer when off axis. The same LED (picture 1 and 4) after the "treatment": it is slightly dimmer when viewed right in front but it is much brighter when it is off axis and it gives a much better overall appearance. Picture 3 and 6 refer to a standard diffused LED and as one can clearly see, it is just too dim. The white part of the picture is where the light is most intense and full of infrared light. As most digital cameras are quite sensitive to infrared light, it is recorded as a white area. This is not really a circuit but I thought to share it with you and unless you need the extra brightness of a front shining LED you may use this trick with any clear LED, blue LED's being especially attractive. 


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