Another two-tone doorbell using a 555 timer oscillator
I picked up this kit on special for $0.66. It even came with extra components, so a bit of a bargain impulse-buy.
The kit turns out to be a simplified version of the TwoToneDoorbell circuit I've built before.
It lacks the power control circuit, and draws about 4mA when idle - so if you actually use it, expect to be replacing batteries on a regular basis.
The basic operation is similar. When S1 pressed:
- 555 timer reset (4) is pulled high and charges the R4/C1 RC circuit
- it bypasses R1 (via D2) so that the 555 a-stable behaviour is governed by R2/R3/C2
- speaker is driven with a 102.128 Hz square wave
When S1 released:
- R1 is added to the 555 a-stable biasing
- speaker output modultes to 76.596 Hz
The duration of the "dong" depends on the R4/C1 RC time constant (𝛕), 470ms. When the voltage on pin 4 falls below the reset threshold (typically 0.5V), the oscillation stops.
Here are some traces of the circuit in operation with 3 channels on a scope:
- CH1: monitoring the C2 charge (pin 2, 6 input)
- CH2: monitoring the pin 4 reset charge/discharge
- CH3: monitoring the pin 3 output
Here's a trace of the initial trigger shwoing the reset line coming high, and the oscillation kick-off:
And on a wider scale - though hard to see the details - here's a complete "ding-dong" cycle. The main point to note is the reset discharge over about 1.2s before the 555 shuts off:
- LM555 Datasheet
- Doorbell DIY Kit - from a seller on aliexpress
- eBay Cheap: $0.75 Electronic Doorbell Kit NE555 - Julian Ilett's build
- ..as mentioned on my blog