Relay Circuits

You can use that circuit to drive a relay of 12 V with a trigering signal of 5V. It uses 2 diodes 1N4002, 1 transistor 2N3904 and two resistors. By changing the resistors value you can change the input triggering voltage...

Monostable to bistable relay with momentary switch

Monostable to bistable relay with momentary switch

This circuit could be used to drive a monostable relay by a singol momentary switch. Powered by 24 Volts it works for a 12 Volts relay. When the button is pressed Q1 is energized and the capacitor charges, while the relay coil is shorted. Once the button is released the relay turns on. Note that Q1 is powered by the capacitor until the relay switch (10-6) closes...

Infrared light trap circuit

Infrared light trap circuit

This infrared light valve has more parts, so it works better and more reliable for alarms. The circuit responds less ambient light. The light valve consists of a transmitter and a receiver. The transmitter consists of a NE 555 which provides pulses to the infrared LED (D1) are fed. The frequency of the pulses is set to P1. On the receiver side we see the photo transistor T1 receives the pulses from D1. The pulses through C4 to the positive input of op amp IC2 led. Between the output of IC2 and the minus input is an RC network that ensures the lowest at about 7.2 kHz amplitude issues. ..

Delay switching for power amplifiers

Delay switching for power amplifiers

This delay can be used in power amplifiers, to prevent the fuses fail when the amplifier is turned on. The circuit is very simple with a relay which turned on when C2 and C3 are charged. If the relay has not been applied, the resistors used to control connected to feed. If after some time the relay attracts the bridge resistors. Watch out for your own safety when building this circuit. On the whole circuit is power. F1 should be the value of having the fuse that normally would be used...

Logic level relay controller

Logic level relay controller

The schematic below illustrates 4 methods of controlling a relay with a digital logic signal. Figure (A) can probably be used in most cases where the relay coil requires 100 mA or less and the input current is 2 milliamps or more. The resistor value (R) is determined from the input voltage and the available current. For example, a 5 volt input signal supplying 2 milliamps would require (5-.7)/.002 = 2150 ohms, or a 2.2K standard value. If the transistor has a minimum current gain of 50, there will be 100 mA of current available for the relay coil. ..

MOSFET Toggle Relay

MOSFET Toggle Relay

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. ..

Parallel Port Interface with relay

Parallel Port Interface with relay

Below are three examples of controlling a relay from the PC's parallel printer port (LPT1 or LPT2). Figure A shows a solid state relay controlled by one of the parallel port data lines (D0-D7) using a 300 ohm resistor and 5 volt power source. The solid state relay will energize when a "0" is written to the data line. Figure B and C show mechanical relays controlled by two transistors. The relay in figure B is energized when a "1" is written to the data line and the relay in figure C is energized by writing a "0" to the line. In each of the three circuits, a common connection is made from the negative side of the power supply to one of the port ground pins (18-25). ..

Relay Toggle with 556 schematic

Relay Toggle with 556 schematic

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. ..

1 Transistor Toggle Relay schematic

1 Transistor Toggle Relay schematic

The circuit below requires a double pole, double throw relay in conjunction with a single transistor to allow toggling the relay with a momentary push button. One set of relay contacts is used to control the load, while the other is used to provide feedback to keep the relay activated or deactivated. Several push buttons can be wired in parallel to allow toggling the relay from different locations. In the deactivated state, the relay contacts are arranged so the 1000 uF capacitor will charge to about 2.7 volts. ..

Bistable Flip Flop with relay

Bistable Flip Flop with relay

Here are two examples of bistable flip flops which can be toggled between states with a single push button. When the button is pressed, the capacitor connected to the base of the conducting transistor will charge to a slightly higher voltage. When the button is released, the same capacitor will discharge back to the previous voltage causing the transistor to turn off. The rising voltage at the collector of the transistor that is turning off causes the opposite transistor to turn on and the circuit remains in a stable state until the next time the button is pressed and released. ..

Mono stable with 555 and relay

Mono stable with 555 and relay

The two circuits below illustrate using the 555 timer to close a relay for a predetermined amount of time by pressing a momentary N/O push button. The circuit on the left can be used for long time periods where the push button can be pressed and released before the end of the timing period. For shorter periods, a capacitor can be used to isolate the switch so that only the initial switch closure is seen by the timer input and the switch can remain closed for an unlimited period without effecting the output. ..

Delaying Pulse from 555 circuit

Delaying Pulse from 555 circuit

The circuit below illustrates generating a single positive pulse which is delayed relative to the trigger input time. The circuit is similar to the one above but employs two stages so that both the pulse width and delay can be controlled. When the button is depressed, the output of the first stage will move up and remain near the supply voltage until the delay time has elapsed, which in this case is about 1 second. The second 555 stage will not respond to the rising voltage since it requires a negative, falling voltage at pin 2, and so the second stage output remains low and the relay remains de-energized. ..

AC current Detector schematic

AC current Detector schematic

This circuit will detect AC line currents of about 250 mA or more without making any electrical connections to the line. Current is detected by passing one of the AC lines through an inductive pickup (L1) made with a 1 inch diameter U-bolt wound with 800 turns of #30 - #35 magnet wire. The pickup could be made from other iron type rings or transformer cores that allows enough space to pass one of the AC lines through the center. Only one of the current carrying lines, either the line or the neutral should be put through the center of the pickup to avoid the fields cancelling. ..

Light Activated switch with Relay

Light Activated switch with Relay

This is same circuit as above with the addition of a photo resistor to trigger the flip flop instead of a push button. The bias resistor in series with photo resistor was chosen so that sufficient voltage is present at the base of the 2N3904 to supply current to the circuit in ambient lighting conditions. ..

Who's First" game lights

Who's First" game lights

The circuit below turns on a light corresponding to the first of several buttons pressed in a "Who's First" game. Three stages are shown but the circuit can be extended to include any number of buttons and lamps. Three SCRs (silicon controlled rectifiers) are connected with a common cathode resistor (50 ohm) so that when any SCR conducts, the voltage on the cathodes will rise about 7 volts above the voltage at the junction of the 51K and 1K ohm resistors and prevent triggering of a second SCR. When all lamps are off, and a button is pressed, the corresponding SCR is triggered due to the voltage at the divider junction being higher than the cathode. ..