What is it?

A debouncer is used in a circuit to ensure a single ON action from a switch. Most switches have a tendency to bounce which can lead to multiple inputs to a digital circuit. This circuit makes sure that an input only receives a single pulse when the switch is depressed.

At first glance it might seem that when the switch is depressed, contact is made in one action. This isn’t usually the case. Because the switch is made from metal parts which have a degree of springiness about them, when a switch closes it can momentarily bounce rapidly back and forth before settling down. This is called switch bounce. In many applications this doesn’t matter. If you’re using a switch to turn on a light, we don’t notice the bounce since it is so fast. If you are trying to control a digital circuit, the bounce can be very bad because the circuit that is connected to the switch will often interpret the bounce as a series of on/off changes and respond accordingly. For example, a bounce can last between 10 and 300 µs. Note that a simple computer like an Arduino is capable of processing a few hundred instructions in that time. To the Arduino or any logic circuit, the bounce looks like a series of on/off input signals which can play havoc with your design.

NOTE: This is a kit that requires basic soldering skills.

Where can I use it?

You can use the debouncer on any circuits we supply and in principle any logic circuits that use 5v standard supply, for example standard TTL circuits. It should also work on inputs to a standard Arduino (untested) which is also at 5v. The input pins on the Raspberry Pi are 3.3v (unlike Arduino which are 5v) so there is no guarantee it will work at this level since you'll have to drop the supply voltage below the operating mimimum for the NE555. It might be possible (untested) to swap the NE555 with a CMOS version LM555CN which can operate at much lower voltages.

Alteratively for the Raspberry Pi it is possible to also use a voltage shifter, such as the SparkFun Logic Level Converter and connect the debouncer to that but this might be overkill.

I originally built the circuit in order to provide bounceless inputs to a binary counter and transistor based logic circuits.

Why did you make it?

I needed a clean signal from a momentary switch that could be used to apply inputs to transistor based logic gates. I also wanted something that could either be plugged into a breadboard and used as a standalone device.

Design

The debouncing circuit uses a standard NE555 as a monostable timer. When the switched is pressed, the output rises to a HIGH value. At the same time an internal timer is activated. Once the timer completes, the output falls back to the LOW settings. In the kit I supply, a capacitor/resistor pair sets the duration of the time to about 100 ms. Any bounce is ignored during the operation of the timer. If longer or shorter output pulses are required then the values of the capacitor/resistor pair can be changed. Details of the simple calculation needed to compute the capacitor/resistor pair are provided in the manual as well as much more detail on how the circuit operates.

Specification:

• Board Dimensions: 3.5 cm x 2 cm
• Input voltage: 4.5 to 9 Vdc depending on application
• Maximum current draw: 5 mA

What's in the box?

• PCB Board
• NE555 timer and socket
• Two resistors
• Three capacitors
• Tactile Tact Push Button Switch
• Three pin connector
• Eight page assembly and how it works manual

NOTE: This is a kit that requires basic soldering skills.