Three Doorbells For The Hearing Impaired
When the push switch is operated – the buzzer
will sound and the LEDs will begin to flash. For the hearing members of the household – the buzzer acts as a regular doorbell. It also re-assures the visitor that the doorbell is working.
When the push switch is released the buzzer will stop – but the LEDs will continue to flash. The length of time they will go on flashing is set by the values of R2 & C1. With the values shown in the diagram – the LEDs will flash for a further 30 seconds or so. If you make R2 a variable resistor, you can adjust the time period. If you want longer than 30 seconds – increase the value of C1 or R2.
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.
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.
The Support Material for these circuits includes detailed circuit descriptions – and all the information you need to adapt them to a different supply voltage.