Article : Andy Collinson
This circuit uses a conventional spring loaded mouse trap, available from hardware stores. However when a mouse is caught, the circuit triggers and
transmits an interrupted tone on the commercial FM band to a nearby radio receiver. The transmitter and tone are adjustable so it is possible to
build more than one unit and monitor several mouse traps simultaneously.
This circuit is a small battery powered transmitter which is activated by vibration from a mechanical mouse trap. This can be done using a microswitch
positioned so that the circuit is off (not transmitting). When the mouse trap is triggered the microswitch lever is moved and activates the transmitter.
The transmitter is a standard hartley oscillator, designed to transmit across the FM band approximately, 87 to 108MHz. The transmit frequency is adjustable
by means of trimmer C8. The combined capacitance of C4 plus C8 and L1 set the resonant frequency.
Resonant Frequency Calculator
The following equations calculates the resonance, where C is the combined valued of C4 plus C8, and L1 is approximately 0.15 uH
Resonant Frequency Calculator
IC1 is a CMOS quad 2 input AND gate. Gates U1a and U1b create a square wave astable oscillator ofabout 2Hz. The output of gate 1b is wired to
the input of gate 1c. Similarly gates 1c and 1d form another astable oscillator, this time about 2kHz. The 2kHz oscillator is activated only when
gate 1b is high, all gates forming and interrupted oscillator. The tone of this oscillator is adjustable via the preset R2.
The output of the oscillator now directly drives the transmitter comprised of Q1 and associated components. The transmitter will be on when the output from
gate 1c is high and off when gate 1c is low. As the input signal is a sqaure wave, there will be some cross modulation and the transmitter signal will
be heard at more than one point across the FM band. As the output power is limited, this is not really a problem and with just 5 inches of wire as an
antenna, I could hear the transmitter with an ordinary radio 15 metres away. The antenna is connector to Q1 collwctor and will work with just a few
inches of wire. A telescopic whip may be used instead. Do not use lengths of wire longer than 30 inches as these will detune the oscillator and
the circuit may not transmit at all.
Construction of L1
L1 is 7 turns of 20swg wire wound on a 6 mm drill bit. This forms an inductor of approximately 150 nH. The value of the inductance can be altered
by compressing or expanding the turns.
Start with a 5 inch length of 20swg enamel
coppered wire. Scrape about 3mm of the insulation from one end with a penknife or sandpaper.
Hold one end of the wire against the drill bit and start to wind the turns clockwise. Continue until all 7 turns have been completed,
and the coil looks like the image of the right hand side. Trim any excess wire so both ends are same length and scrape about 3mm of insulation
off the opposite end.
If preferred commercial coils may be used. Some have moulded cores and adjustable ferrite slugs to alter the inductance, as shown (right).
. It is presented here again for
convenience. For example using a diameter of 6mm (0.6cm) and 1cm length, a desired inductor of 0.15uH works out at 7.33 turns.
Monitoring Multiple Mouse Traps
An added feature is that multiple mouse traps can be monitored. Simply build more identical circuits and tune them to the same transmitting (RF) frequency.
R2 must be set different in each case so that a different note is heard for each mouse trap. If two traps activate simultaneously then a "beat" note will
be heard which will be the sum of two different notes.
Alternatively, each trap may be set to a different RF frequency and different radios used to monitor the mouse traps.
Picture of my Prototype
Finally a picture of my prototype on breadboard. Breadboard and veroboard have adjacent track capacitances of around 0.2pF. This may not sound
much but limits the high frequency response of any circuit. Although construction on veroboard is possible either a PCB layout or copper clad
board and copper island construction preferred.
The home made coil may be seen across contacts B1 and C5, the antenna wire is connected to terminal D6.
Ref Description Value
C1 -- polyester capacitor 220nF
C2 -- ceramic capacitor 10nF
C3 -- electrolytic capacitor 100µF
C4 -- ceramic disc capacitor 15pF
C5 -- ceramic disc capacitor 6.8pF
C6 -- ceramic disc capacitor 10nF
C7 -- polyester capacitor 100nF
C8 -- -- trimmer cap 25pF
L1 -- -- inductor, 150nH
Q1 -- -- transistor BC109C
R1 -- -- resistor, 2.2M
R2 -- -- Preset resistor 220k
R3 -- -- resistor, 10K
R4 -- -- resistor, 270R
U1 -- -- CMOS IC CD4011B
Below is a working prototype built by Arturo Dy Buco from the Phillipines. All images zoom when clicked. The that is set with cage door open.
A mouse entering the trap causes the door to close, the lever on the microswitch is positioned so that the transmitters activates when the
cage door has closed. The mouse is unharmed and can eat cheese someplace else.