Clap Switch circuit schematic

Clap Switch Using Latching Relay

This is a simple clap switch circuit with high sensitivity. The only unusual component is the single coil latching relay. While there are numerous means of generating the latching function, the latching relay greatly simplifies the circuit and provides a relay contact interface for driving a 115 or 230VAC load such as a light bulb.

Battery life is good since the load is only 400uA –there is no relay coil current in either the On or Off states. The “clap” signal is picked up via a computer speaker that is utilized as a microphone, amplified by a high voltage gain audio amplifier and detected via a simple, but sensitive transistorized floating level detector (comparator). The high voltage gain amplifier will be discussed in a future article. The floating level detector portion of the circuit may be new to the world as I have never seen anything like this published. All circuitry consists of discrete components so it may be easily simulated.

Schematic of the Clap Switch Circuit

Clap Switch circuit schematic

Single coil latching relay

Every experimenter needs one of these latching relays in his bag of tricks –inherent is an electromechanical memory property that remembers its state indefinitely in the absence of power. I cheated here because I did not have one on hand and tested the circuit with a standard non-latching relay type. It is permissible to use a 5V relay on 9V because the coil impulse lasts for only 80mS. These relays are relatively inexpensive –following are two screen shots; one available on eBay and the others from DigiKey. Note that these are low power devices that may be used to interface to low power 115 and 230VAC circuits (e.g. 60W light bulb etc.). DigiKey also has a selection of single coil latching power relays that handle >10A –I will let you do the search for these.

eBay screenshot

latching relay from eBay

Latching Relay ebay

Also pull up the spec sheet for this device for contact rating:

Digikey screenshot

latching relay selection

Latching Relay Selection

High voltage gain amplifier

Since the output of the computer speaker is only 200uV or so, a high voltage gain is required. Q1 & Q2 comprise the amplifier –its output is at the emitter of Q2. The only DC load to the circuit is what flows through R5 (about 400uA). The voltage gain is in the order of 1000 or so –it is more than adequate to operate the detector, and the sensitivity adjust pot (R10) must be turned down to prevent oscillation –the speaker picks up the sound of the relay operation. For this reason, it may be desirable to separate the relay from the speaker.

Battery operation

Battery operation is mainly for experiments –any real application should use a wall wart power supply. 12V should also work fine.

Floating noise detector

The noise detector is novel, consisting of only one PNP transistor and a few passive components –no adjustment required. The emitter of Q3 is isolated from the output of the amplifier via an R-C circuit that slowly tracks the DC voltage coming out of Q2 –C4 performs the function of providing a low impedance for transients (bypass capacitor) so that its voltage varies little as the collector of Q3 charges C3. A PNP bipolar transistor generally has a threshold of -0.6V. To reduce this voltage, voltage divider R7 & R8 pre-biases the base of Q3 at about -0.4V –the resulting sensitivity is about 200mV. To make it less sensitive, R8 may be eliminated –I found R8 unnecessary.

Relay driver

The relay driver (Q4) consists of a 2N7000 MOSFET transistor. Its extremely high input resistance is easy driven by the low-power noise detector. R9 is sufficient to discharge C3 in about 80mS to the Vgs threshold of Q4 (2V).

Bipolar Transistors

Recommended are the high gain BC547C /BC557C types, but there are many others that should work OK. In the circuit, I actually used MPSA15 (NPN) and 2N5087 (PNP) transistors.


clap switch oscillographs

I had another interesting oscillograph with a 5mS /div timebase, but lost it due to buggy Tektronix Wavestar software –never, never turn off the oscilloscope before saving the file, nor save the file upon exiting the program or you will never read it again –thanks Tektronix! The lost oscillograph showed C3 charging up in steps for each cycle of the clap signal.

Clap Switch Photos

For the future

Clap switch that discerns brief claps from loud continuous signals –requires logic chips
High voltage gain amplifier


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