Kids' Basics: Bedroom Doorbell

Using a 555 Timer

Daniel Koch

Issue 38, September 2020

Sure you can ask your family to knock at your bedroom door, but how much more important do you feel when you have your own doorbell? Build this one and find out!

This simple build makes use of one of Kids’ Basics’ best friends: The 555 timer integrated circuit (IC). We’ve used this chip in several projects before. It has flashed lights, detected metal, made musical drawings and sounded a siren to go with the flashing lights. It has also been a clock pulse generator for projects involving decade counters. Now we are going to use it to make two different sounds with one circuit, to produce a ‘ding-dong’ sound reminiscent of an old electric-chime door bell.

As usual with Kids’ Basics, the only assumed knowledge is that you can identify basic components. We’ll give the resistor colour codes, and any other relevant codes like the multipliers on MKT capacitors. Electrolytic capacitors have their values written on the case. We never solder in Kids’ Basics, or use specialised tools.

If this is your first project, you may also like to read “Breadboard Basics” in Issue 15, to familiarise yourself with using a breadboard before getting started. Knowing how to read a circuit diagram helps but is not necessary.

The Build:

Parts Required:JaycarAltronicsCore Electronics
Solderless BreadboardPB8820P1002CE05102
Breadboard Wire LinksPB8850P1014ACE05631
4 x Plug-to-plug Jumper Wires *WC6024P1022PRT-12795
1 x 120Ω Resistor *RR0550R7536COM-05092
1 x 820Ω Resistor *RR0570R7556COM-05092
1 x 8.2kΩ Resistor *RR0594R7580COM-05092
1 x 9.1kΩ Resistor *RR0595R7581COM-05092
1 x 91kΩ Resistor *RR0619R7605COM-05092
1 x 110kΩ Resistor *RR0621R7607COM-05092
1 x 50kΩ PotentiometerRP7516R2227A-
1 x 18nF MKT or Ceramic Capacitor *RM7080R3016A-
1 x 22μF Electrolytic Capacitor *RE6079R5084CE05130
1 x 100μF Electrolytic Capacitor *RE6130R5123CE05130
1 x 1N4148 or 1N914 Small Signal Diode *ZR1100Z0101COM-08588
1 x BC547 NPN Transistor #*ZT2152Z1040-
1 x NE555 Timer IC %ZL3555Z2755002-512-LM555CN
1 x Pushbutton SwitchSP0710S1060BCE05249
1 x Small SpeakerAS3000C0610ADA1890
1 x 9V Battery SnapPH9232P0455CE05205
1 x 9V BatterySB2423S4970BCE05337

Parts Required:

* Quantity shown, may be sold in packs.

# BC547 used in project, but BC548 and BC549 or any other transistor with the same specifications and pin-out can be used.

% Any 555 will work. Some are NE555, some LM555, and others exist as well.

Step 1:

Position the breadboard in front of you with the outer red (+) rail away from you and the outer blue (-) rail closest to you. Insert the 555 timer so the notch or dot showing pin 1 is to the left. Add the two wire links to connect pin 8 to the upper red (+) rail and pin 1 to the lower blue (-) rail. Also insert the two wire links that join the two sets of power rails together, blue (-) to blue (-) and red (+) to red (+).

Step 2:

Add three wire links to join pin 6 of the 555 to pin 2, around the left side of the IC. Also add two wire links to the right. One bridges the gap in the middle of the board, while a small uninsulated one joins that link to pin 4 of the 555.

Step 3:

Add the two resistors at left. The 8.2kΩ (grey-red-black-brown- -brown) goes across the centre gap while the 9.1kΩ (white-brown-black-brown- -brown) goes to the lower blue (-) rail. Add the 1N4148 diode above the 555 and the wire link from the junction of the resistors to a spot to the right of the 555 for later use.

Step 4:

Install a 91kΩ (white-brown-black-red- -brown) resistor from the upper red (+) rail to pin 7 of the 555. Add a 110kΩ (brown-brown-black-orange- -brown) resistor between pin 7 of the 555 and a spot to the right, then add a wire link to connect it to pin 6 of the 555. Also install the wire link shown below the resistors, from pin 5 of the 555 to a spot on the right.

Step 5:

Install a 820Ω (brown-red-black-black- -brown) resistor between pin 4 of the 555 and the lower blue (-) rail. Add a wire link next to it from the lower red (+) rail. Install a 120Ω(grey-red-black-black- -brown) resistor between pin 3 of the 555 and a spot to the left for later use.

Step 6:

Insert the BC547 transistor with its face away from you so that its middle (base) leg lining up with the wire link installed earlier which connects to the two resistors at the left of the 555. Also install a wire link from the lower blue (-) rail to the left hand leg (emitter) of the BC547.

Step 7:

Insert the 50kΩ potentiometer so that its left-hand leg (when the spindle faces away from you as shown) connects with the wire link from pin 5 of the 555. Add a wire link to connect the middle leg (wiper) of the potentiometer to the left-hand leg, and another to connect the right-hand leg across the gap in the board.

Step 8:

Insert a wire link to join the right-hand (collector) leg of the BC547 to the link from the potentiometer. Install the 100μF electrolytic capacitor with the negative (striped) side to the resistor and the positive side in the same row as the wire link. Note that ours sits over a previous wire link, with the legs beside it.

Step 9:

Insert the 18nF MKT capacitor (18n or 183) between the lower blue (-) rail and pin 2 of the 555. You will have to bend the legs carefully with pliers to make this work neatly. Install a 22μF electrolytic capacitor with the positive side to the 120Ω resistor and the negative (striped) side in the next row.

Step 10:

Cut the ends off four plug-to-plug jumper wires. Bare the cut ends of the wire and twist one through each terminal of both the speaker and the pushbutton switch. Make sure the connections are tight then cover each connection with tape to prevent short circuits.

Step 11:

Insert the speaker so that one wire connects to the negative side of the 22μF capacitor at the right of the board, and the other wire in the lower blue (-) rail. Install the pushbutton switch so that one wire is in the row that connects to the 8.2kΩ resistor and 1N4148 diode, and the other is in the upper red (+) rail.

Step 12:

Connect the battery clip and install a battery. Pressing the pushbutton should give a ‘ding’ sound for as long as it is held. As soon as you let go, the ‘dong’ sound starts, for a time set by two components as we’ll see shortly. If you don’t hear a sound, disconnect the battery and check all connections.

What’s Going On?

Previously, we have set the timing frequency of the 555 and left it that way. Pin 4, the reset pin, is almost always tied to the power supply. We also usually tie pin 5, the control voltage pin, to ground via a small (100nF or so) capacitor. This circuit is different. Generally, the frequency of a 555 is controlled by the rate of charge and discharge of the capacitor connected to pin 2, the trigger pin. In this case that is C2, fed via the 91kΩ and 110kΩ resistors R4 and R5. That still happens at a certain point in this project, but it’s not the first sound you hear. When the button SW1 is first pressed, current flows via D1 to pin 4, the reset pin of the 555. This takes the reset pin high but also discharges the 100μF capacitor C1.

While the 555 is now oscillating, you don’t hear the frequency generated in this state, because current also flows to the base of Q1, the BC547, via the 8.2kΩ resistor R1. This turns on the transistor, which has the variable resistor VR1 connected between the transistor’s collector, and pin 5 of the 555. Pin 5 is the control voltage pin, and varying the resistance to ground here alters the voltage thresholds at which the 555’s internal flip-flop resets, which in turn alters the frequency. This defines the sound you first hear, the ‘ding’ sound. The resistance set by VR1 (combined with a minor amount of internal resistance through Q1) sets this frequency, which is why we used a variable resistor made by connecting the wiper of a potentiometer to one of its ends.

As soon as you release the pushbutton, the 100μF capacitor C1 charges via the 820Ω resistor, which is connected to ground. Current will only flow through this to pin 4 while the capacitor is charging, so when it is charged, pin 4 is no longer held high, and the oscillation stops. This is the end of the ‘dong’ sound. At the same time as that process begins, current is no longer provided to transistor Q1, which turns off. This isolates pin 5, removing the voltage control pin’s effect from the oscillation, which is why the tone changes.

The 555 is now oscillating at the frequency set the way we are familiar with: The capacitor C2, resistor R4 at 91kΩ and resistor R5 at 110kΩ.


There are two different directions for experimentation from this point. One is electronic, the other physical.

Electronically, you can change the values of almost any of the components to see what the effect is on the circuit. The main components to alter are C2, C1, R4, and R5, but changing C3 will gain some effect as well. R3 can also be changed, but leave R1 and R2 as they are. R6 only limits current to the speaker so leave it as-is to avoid any change of overloading the 555’s output.

Change components one at a time, and do so incrementally. For example, of changing 110kΩ resistor R5, start with 100kΩ or 91kΩ, don’t jump straight to 10kΩ. The same goes for capacitors, although the increments are often larger. For example, capacitor C1 is 100μF, but the range in this series is 10μF, 22μF, 33μF, 47μF, 100μF, then there is a jump to 220μF, and 330μF.

Physically, you might like to mount your creation into a box so it is more user-friendly. Try a small cardboard gift box and mount the switch and speaker in holes in the lid. You might also use a take-away food container or something similar. Make sure you get adult help with any cutting or blade work.