Doorbell

The purpose of the doorbell is to evaluate your soldering and learn how it works. It uses a well-know 555 timer chip and a speaker. If you press the button, it make a standard doorbell noise. If you understand the design, you can change this.

DoorBell Kit

Cheap kits ($1.20) from AliExpress is ideal for solder practice and assessments.

Package Includes:

1* 8 Ohm 0.25W speaker
2* 100MM red line
2* 100MM black line
1* NE555 chip
1* 8P IC socket
1* 100UF 25V Capacitor
1* 10UF 25V Capacitor
1* 6*6*5 pushbutton switch
3* 103 ceramic capacitors
5* 47K resistor
2* 1N4148 diode

Completed

Specifications:

Package Name: NE555 doorbell kit
Working voltage: 4.0-7.0V
PCB size: 28mm (L) *25mm (W)
Kit horn: 8 Ohm 0.25W
Color: Beige
Net Weight: 15g

Schematics

When doorbell switch s1 is pushed, the speaker will make a sound. Without being pressed, C1 is not connected and cannot be recharged, and capacitance voltage is 0 v, the reset has been in a low level condition, and there is no output to speaker (No sound).

When switch s1 is closed, the VCC current goes through the diode D1 to charge capacitor C1, and reset voltage also began to rise gradually. When the voltage of the capacitance increased to higher levels, namely the reset input port is the high level, the timer starts, based on resistance and capacitance and oscillator starts working at output frequency of f1.

When disconnecting switch s1, resistance and capacitance circuit, capacitor began to discharge through R4. At the same time composed of resistance, capacitance and diode oscillator start work, output frequency of 2 f. Restore low level is reset when the capacitor discharge end, timer stop working.

In the speaker output termination, output current will make the speaker sound. When output frequency is not at the same time, the sound is different. This circuit has two kinds of different frequencies, so the speaker will make a "ding" "dong" sound.

The 555 Timer

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element - https://en.wikipedia.org/wiki/555_timer_IC. This website has detailed info and circuits about timers. There are three modes: Monostable, Astable and Bistable. These are described here. The resistor and capacitor values can be determined using the 555 calculator. There is one mistake though with one of the formulas but the calculation is correct. See if you can find it. There are lots more info about these timers on the internet.

To improve our understanding of the NE555 timer IC we will look at the different modes it can be used in. These are simulations in Yenka.

AStable Operation

This mode are used to generator a square wave. Enter this circuit in Yenka and test.

Monostable operation

This mode is use to trigger an event that will turn off after a certain time. Enter this circuit in to Yenka and test.

bistable operation

This mode turn the output on when the trigger is reset and turn the output off when the reset is tuned low. Enter this circuit in to Yenka and test.

Which mode is used in the doorbell?

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