Light Dark Sensor With Relay Circuit Using LM741

Above is a schematic diagram of an LM741 light/dark sensor circuit (from the excellent 741 Op-Amp Tutorial by Tony van Roon).
The ECG128/NTE128 transistor stipulated can be substituted with any NPN transistor rated at sufficient gain and current for the chosen relay coil.

1st Nov 2007 Update - We have modified the schematic diagram above with the addition of a 220uF smoothing capacitor between the base of transistor Q1 and ground. Without this capacitor, the relay chatter (relay switching on and off many times per second) was terrible around the switch on/off light level. By adding the capacitor, relay chatter was completely eliminated.

According to the designer of this circuit, the relay will be closed only when "NO light falls on LDR1", however, in testing this circuit proved to work very well with the user able to adjust the potentiometer (P1) to automatically close the relay at whatever light level they chose.

By swapping the postitions of the 10K resistor (R1) and the LDR (LDR1), the relay will be closed when the LDR is under light rather than under darkness. Therefore a device can automatically be switched off at nighttime.

Since this circuit still contains a relay we need to make some changes* to reduce the amount of power to make it more suitable for renewable energy powered low-current applications.

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BC108 Water Activated Relay Circuit

Water Activated Relay circuit

For electronic circuit has two transistors wired as a pair, together with high gain.

2N2222A transistor T1 and T2 is BC108.

The actual profit will be the proceeds of any beta-transistors with a minimum of 140 x 110 or 15,400.

Power supply is a voltage from 4.5 to 15 volts, a kind of relay 5 Volt at 60 mA is required to in this case, any liquid with a stream of at least 4 others to the relay. It is easy to reach with tap water or rain.

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IC 555 Time Delay Relay Circuit

Time Delay Circuit

In the design of analog circuits, there are times when you would need to delay a pulse that came into a circuit before being used for the next process. This time delay circuit uses a 555 timer to delay a pulse that comes in to a maximum time of 75 seconds. The timing of the delay can also be changed by changing the resistor value of VR1 and the capacitor value of E based on the time delay formula of t=0.69RC.

In order for the output to go high, the reset pin of 555 timer (pin 4) must be high and the TRIGGER pin (pin 2) voltage level must be below a third of the level of the power supply to the IC. When there is no pulse being applied to the input, transistor Q1 will turn ON and capacitor E is charged.

IC 555 Time Delay Relay Circuit

Once a pulse is applied to the input, transistor Q1 will turn OFF and pin 4 reset pin is held to high. This caused the capacitor E1 to be discharged through VR1 resistor. The time delay will depend on the discharged of capacitor E to a third of the supply before the output of 555 goes high. Experiment with different values of VR1 and E to get different time delay.

If the maximum value of potentiometer is set to 5M ohm, the time delay of the pulse will be 75 seconds.

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Video Activated Relay Circuit

If you want the tuner you need to build the tuner by yourself ? The relay contactor output was connecting to electronic circuit by under control. What is that mean? Can You directly connect the relay to a socket ? and the TV can straight away plug in to the socket? For the electronic circuit diagram which is only DC voltage , but i think the tv need AC , Yes?

So,do you need to put a transfomer ? where should you put ? how to decided which kind of transformer to be used?

We think normally is 12Volt or 9Volt transformer? Yes.

Electronic Component Part List:

1. R1, R2 10K 1/4 W Resistor
2. R3 1K 1/4 W Resistor
3. R4 33K 1/4 W Resistor
3. C1 1uF Electrolytic Capacitor
4. Q1, Q2, Q3 2N2222 NPN Transistor 2N3904 NPN Transistor
5. D1, D2, D3 1N4148 Diode
6. K1 9V Relay
7. J1 RCA Jack

Remark:
1. For you will be take the circuit to switch for control mains voltage, it should be enclosed in a case.

2. This electronic circuit may be also work with most line level audio, however you may have to change the resistance value of R1 .

Video Activated Relay Circuit for apply your electronic project

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BC108 Water Activated Relay Circuit

For electronic circuit has two transistors wired as a pair, together with high gain. 2N2222A transistor T1 and T2 is BC108. The actual profit will be the proceeds of any beta-transistors with a minimum of 140 x 110 or 15,400. Power supply is a voltage from 4.5 to 15 volts, a kind of relay 5 Volt at 60 mA is required to in this case, any liquid with a stream of at least 4 others to the relay. It is easy to reach with tap water or rain.

Read More → BC108 Water Activated Relay Circuit