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Single button ON-OFF toggle with discrete transistors

     This circuit allows the switching appliances (loads) on and off with one momentary button. Once you press it, it turns on, press again to turn off. Usually this is built with binary divider or 555, but the disadvantage is the current consumption in OFF state and that this way is complicated. So I designed circuit without integrated circuits, with transistors only. In the off state it draws no current, because both transistors are closed. It is therefore suitable for battery power.
     The working principle is simple: In OFF state the C1 is charged through the load and R2. After pressing the TL1, the voltage of C1 connects to T2 gate, it opens and turns on the load. At the same time T1 opens via R1 and further maintains the positive voltage on gate. C1 discharges through R2 and T1. When TL1 pressed again, gate of T2 is discharged into C1 (C1 has a much higher capacity than the gate). This closes the T2 and then T1. Resistance R3 keeps gate at 0V and the circuit remains in the OFF state until the next pressing of TL1.
     As a switching element T2, N-type MOSFET was chosen, because it allows very little loss and in idle state its gate does not draw any unnecessary current. You can use any low-voltage MOSFET with Uds about 20 - 55V. The smaller on-state resistance, the better. Transistors rated at unnecessarily high voltages usually have the greater resistance. I used IRF3205 to test the circuit. You can also use MOSFETs from the PC motherboard. T1 is any small PNP like BC327, BC557 or 2SA733. In case of problems, ringing or too much capacity of gate, increase the value of C1. Maximum supply voltage is limited mainly by max voltage Ugs of T2 and maximum current by his allowable loss. The minimum supply voltage depends on the minimum voltage that turns T2 completely on. It is usually lower in LOGIC mosfets. In the case of inductive loads connect antiparallel diode to the output. When the load has a small current or its own switch, connect a parallel resistance of about 100k. The circuit is not suitable for loads with parallel capacity (modification would be necessary).

The schematic of Single button ON-OFF toggle with discrete transistors.

PCB board pattern and mounting plan, which made Temsi according to my schematic (I thank him). Here are the files for download: board pattern for printing in PDF and mounting plan in Eagle.

Breadboard test of the momentary button ON-OFF switch circuit

Video - demonstration of operation.

Added: 8. 10. 2011