These are a few simple LED doorbell circuits. They can also be used as warning lights and fake alarms.
Doorbell Circuit No.1
The first circuit is described in complete detail. The remaining two circuits are only slightly different from the first - and their descriptions deal only with those differences. Therefore - to understand how the later circuits work - you should begin with Doorbell No.1
Doorbell Circuit No.2
The last circuit will flash up to two groups of 3 LEDs in tandem. This circuit will flash the two groups alternately. The alternate flashing creates the illusion of movement - and makes the display more eye-catching. Note that - although I've drawn the two groups of LEDs side by side - the individual LEDs can be mounted in any pattern you like
Doorbell Circuit No.3
The main difference between this circuit and the last one - is the addition of the two transistor switches. The switches will each flash up to 15 groups of 3 LEDs. And - because they are getting power directly from the battery - the LEDs will glow at their full brilliance.
Changing The Supply Voltage
I've drawn the circuits with a 9-volt supply. However - they can be adapted to a 6-volt or a 12-volt supply. This may involve changing the number of LEDs in the groups - and the value of Rx will need to be re-calculated. The following example is for Circuit No.3.
The LEDs have a forward voltage drop of about 2v each. With the 9-volt supply and three LEDs - you have 6-volts across the LEDs and 3-volts across the resistor. The current flowing through the three LEDs is the same as the current flowing through the resistor. And the current flowing through the resistor is V ÷ R. That is: -
3v ÷ 180Ω = 17mA.
With the 6-volt supply you should use groups of two LEDs. Then there's a total of 4-volts across the LEDs and 2-volts across the resistor. Change the value of the resistor to 120 ohms - and the current flowing through the two LEDs is: -
2v ÷ 120Ω = 17mA.
With the 12-volt supply you can use groups of five LEDs. Then there's a total of 10-volts across the LEDs and 2-volts across the resistor. Again - with a 120 ohm resistor - the current flowing through the five LEDs is: -
2v ÷ 120Ω = 17mA.
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