#201 555 Timer Doorbell Kit

Another two-tone doorbell using a 555 timer oscillator

Notes

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.

Behaviour

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:

Construction

Credits and References

• 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