Signals & Wireless (RF)
Build a wireless doorbell: tune, amplify, detect, switch
A wireless doorbell is a one-bit radio link. Pressing the button switches on a carrier, here 8 V at 20 kHz, and across the garden a receiver must tune to that carrier and ignore a neighbour's, amplify back what the path lost, detect the carrier into a steady level, and switch a chime while that level is up. Each stage makes one decision.
Stage 1: pick the station
A parallel LC tank from the antenna to ground resonates where the coil's and the capacitor's reactances are equal. There its impedance is high and the signal passes; anywhere else one part's reactance is small and shorts the signal to ground. The neighbour's transmitter is on 30 kHz, never stops, and arrives just as strong as your bell. With a 100 µH coil:
281 nF would tune it to the neighbour's 30 kHz instead.
Stage 2: win back the path
The path across the garden loses 40 dB, a factor of 100 in voltage. A non-inverting op-amp with Rg = 1 kΩ and Rf = 99 kΩ gives 1 + 99 = ×100, +40 dB, so the carrier arrives back at about the 8 V it was sent with.
Stage 3: detect and hold
A diode charges a hold capacitor on each carrier peak, and a bleed resistor drains it. The hold voltage sits on a MOSFET's gate, and the MOSFET switches the chime once the gate passes about 3.5 V. A 220 nF hold capacitor fills within a few carrier cycles while the button is held, and drains through the bleed soon after it's released.
The timing trade
The hold capacitor sets how fast the chime answers. Ten times the capacitance, 2.2 µF, fills ten times more slowly: during a 0.8 ms press it reaches only about 1.5 V, and the chime never lights. Too small, and the gate voltage sags between carrier peaks.
Live experiment
Test the doorbell receiver
Your bell sends 20 kHz, the neighbour's transmitter sends 30 kHz all the time, and the path loses 40 dB. Pick the tank capacitor, Rf and the hold capacitor, then test: only 633 nF, 99 kΩ and 220 nF keep the chime dark for the neighbour and lit for your bell.
“Project: Wireless doorbell” is the project that ends Airwave Atoll, Signals & Wireless (RF) World 3. Free with an account. Lessons unlock in order, so you’ll do the 20 before it first.
What real doorbells do
Shop-bought wireless doorbells use the same plan at 433 or 868 MHz. The button sends on-off keying carrying a short code rather than one bit, a data slicer in the receiver turns it into bits, and a microcontroller checks the code before it plays the chime. That's how your bell ignores the neighbour's button even on the same frequency.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Project: Wireless doorbell” is the project that ends Airwave Atoll, Signals & Wireless (RF) World 3. Free with an account. Lessons unlock in order, so you’ll do the 20 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 633 nF. 1 ÷ (4π² × (20 kHz)² × 100 µH) ≈ 633 nF. 281 nF would tune it to the neighbour's 30 kHz.
- Why not “281 nF”? 281 nF tunes 100 µH to 30 kHz: the neighbour's station.
- Why not “63 nF”? 63 nF is a tenth of the right value and tunes it to about 63 kHz.
- Why not “1 µF”? 1 µF tunes it to about 15.9 kHz, below the bell's 20 kHz.
Show the answer
Answer: Nearly a short: the signal barely reaches the diode. Away from resonance either the coil or the capacitor has a small reactance, and it shunts the signal to ground.
- Why not “A perfect amplifier”? A coil and a capacitor can't add energy, so they can't amplify.
- Why not “An open circuit”? A parallel tank is open, a high impedance, only at its resonance. Off it, one part's small reactance shunts the signal.
- Why not “Exactly 470 Ω”? 470 Ω is the feed resistor in front of the tank, not the tank itself, whose impedance changes with frequency.
Show the answer
Answer: Their reactances are equal. XL = XC, at f₀ = 1 ÷ (2π √(L × C)).
- Why not “The current is zero”? Resonance is defined by the reactances matching. Through a series LC the current is actually largest there.
- Why not “The capacitor is fully charged”? A capacitor charges and discharges on every cycle. Resonance is about frequency, not a state of charge.
- Why not “The coil's reactance is zero”? A coil's reactance is zero only at DC. Resonance is where the coil's and the capacitor's reactances match.
“Project: Wireless doorbell” is the project that ends Airwave Atoll, Signals & Wireless (RF) World 3. Free with an account. Lessons unlock in order, so you’ll do the 20 before it first.
Questions people ask
How does a wireless doorbell work?
The button holds a small transmitter that sends a short burst of radio, usually on-off keyed at 433 or 868 MHz and carrying a code. The chime holds a receiver tuned to that frequency, which detects the burst, checks the code and plays the sound.
Why does my wireless doorbell ring by itself?
Another transmitter nearby, often a neighbour's doorbell or a weather sensor, is sending on the same frequency, and the receiver accepts it. Changing the bell's code or channel, which many models allow, usually stops it.
What range does a wireless doorbell have?
Makers often quote 50 to 300 m in open air, and much less through brick and metal. Range follows the link budget: every doubling of distance costs 6 dB in free space, and every wall takes more.
Related topics
- How to calculate a radio link budgetAdd up every gain and loss in decibels. If what arrives beats what the receiver needs, the link works.
- LC resonant frequency explainedA coil and a capacitor swapping energy at one frequency: f₀ = 1 ÷ (2π√(LC)), tanks and tuning.
- How a radio receiver turns a signal into clean bitsA comparator and a halfway threshold, read in the middle of every bit: rounded radio becomes clean 1s and 0s.
- What is frequency-shift keying (FSK)?A low note for 0, a high note for 1: read bits by pitch, so fading can't fool you.