Relay Toggle Circuit | Electronic Project Circuits

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Single MOSFET Relay Toggle Circuit

This circuit is similar to the one above, but uses a N channel mosfet such as IRF530, 540, 640, etc. in place of the NPN transistor. Smaller mosfets could be used, but I don’t know the part numbers. I tested the circuit with a IRF640, IRFZ44, IRFZ34 and REP50N06. The circuit has the same three advantages, it requires only a few parts, always comes up with the relay deactivated, and doesn’t need any switch debouncing.

In operation, when the relay is deactivated, the 100uF capacitor will charge to 6 volts. When the button is pressed, the capacitor will apply 6 volts to the MOSFET gate turning it on. The capacitor voltage (and gate voltage) will fall from 6 to 3 volts in about 200 mS which should be enough time for the relay contacts to move. For very slow relays, a larger capacitor may be needed.

When the relay energizes, the contacts will apply 12 volts to the 3.3K resistor producing 6 volts at the gate, which will keep the relay energized indefinetly. The capacitor will now discharge to zero since the +12 relay contact is no longer connected to the 15K resistor.

When the button is again pressed, the capacitor will apply zero volts to the gate turning off the relay. There should be no problem holding down the button causing the relay to re-engage since the gate voltage will be only about 1.8 volts when the button is held down and the mosfet requires about 3.5 volts or more to start conducting. But you do need to wait about 1 second or longer between button presses, so the capacitor has time to charge or discharge. Two push buttons are shown, but you could have several more in parallel to control the relay from several different locations.

From : http://ourworld.compuserve.com/homepages/Bill_Bowden/

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Categories: Basic electronics, Digital, Electronic Control Tags: mosfet control circuit, mosfet controlled thyristor, relay toggle circuit

Relay Toggle Circuit Using a 556 Timer

This toggle circuit operates by using a couple 555 timers wired as inverters. Pins 2 and 6 are the threshold and trigger inputs to the first timer and pin 5 is the output. The output at pin 5 will always be the inverse of the input at pins 2 and 6. Likewise, the output at pin 9 of the second timer will always be the inverse of the input at pins 8 and 12. A 100K resistor connects the output of one inverter to the input of the other so the state of one will be the opposite of the other.

In operation, the 1uF capacitor will charge to whatever voltage is present at the output on pin 5. When the button is pressed, the capacitor voltage will be applied to the input of the other timer which will reverse the state of both timers and toggle the relay, either on or off.

To follow it more closely, assume the output at pin 5 is +12 volts and the second output at pin 9 is zero volts. The 1uF cap will be charged to 12 volts. When the button is pressed, the cap will apply +12 to the inputs at pin 2 and 6 which will cause the output at pin 9 to go to zero, turning off the relay. When the button is released, the cap will discharge to zero, since the voltage at pin 5 is now zero. When the button is again pressed, the capacitor will apply zero to pins 2 and 6 causing the output at pin 9 to switch positive and engage the relay, and the cycle repeats.

The advantage of this circuit is the large hystersis range on the inputs. The button can be held closed indefinetly without upsetting the state of the outputs since the input voltage will be 1/2 the supply due to the equal value 100K resistors. The switching points are 1/3 and 2/3 of the supply so that a voltage of 50% has no effect. The circuit will also toggle very fast and needs no switch debouncing. One disadvantage is that it may turn on with the relay either engaged or disengaged. To solve that problem, you could use a resistor in series with one of the reset lines (4 or 10) and add a capacitor from the reset line to ground.

The 100 ohm resistor and 100uF capacitor serve to filter noise on the supply line if the circuit is used in a automotive application. They may not be necessary. The circuit may work well without those parts.

From : http://ourworld.compuserve.com/homepages/Bill_Bowden/