A basic two-tone doorbell using a 555 timer oscillator, with n-channel FET for power conservation.
Here’s a quick video of the circuit in action:
When the button is pressed, it triggers three actions:
- 555 timer reset (4) is pulled high and charges the R4/C2 RC circuit
- it bypasses R1 (via D1) so that 555 astable behaviour is governed by R2/R3/C1
- it pulls Q1 gate high and charges the R5/C4 RC circuit
While the button is down (“ding”), the frequency of 555 astable oscillation is governed by R2=47kΩ, R3=47kΩ and C1=33nF, which results in a frequency of about 309Hz.
When the button is released:
- R1 comes into play, reducing the 555 astable oscillation
- 555 remains triggered while C2 drains through R4
The “dong” frequency of 555 astable oscillation is governed by R1+R2=94kΩ, R3=47kΩ and C1=33nF, which results in a frequency of about 232Hz.
The circuit remains “live” while Q1 permits drain-source current flow. Q1 will remain on while C4 discharges and maintains gate voltage above the gate threshold voltage.
A couple of bypass capacitors minimize the impact of voltage spikes and noise on the operation of the 555 timer, especially during transitions of the timer’s output transistors:
- 100nF (C5) between control pin (5) and ground
- 100nF (C6) across the power supply
OK, this is a pretty crappy doorbell, but just for the heck of it I put the circuit on a protoboard and used as a short-term replacement for a corroded/failed bell.
Credits and References
- 2N7000 Datasheet
- LM555 Datasheet
- Visual 555 Calculator
- Two-tone Doorbell kit - example of a similar circuit as a kit, using a mosfet for switching (looks like a very old site, not sure if still available).
- Two-tone Doorbell kit - another example circuit as a kit; this uses BJT for coupling the output speaker and an RC network to provide timed-on power.