Jun 13

Mole Deterrent Teardown

Having a very healthy population of moles in our vicinity, I logged onto eBay and bought four of the cheaply available (ultra?)sonic mole deterrents for our garden lawns in the hope of persuading them to move elsewhere. Here’s a link to a couple of similar ads: eBay ad, eBay ad2.


The basic idea is pretty simple and used with other animals too – the gadget emits an frequency that the animals in question (in this case the moles) aren’t keen on, so they move away from the area. Some are advertised as being “ultrasonic mole deterrent”, and some simply say “sonic mole deterrent”.

A few questions spring to mind:

  1. Do they actually work?
  2. Are they sonic or ultrasonic? (ultrasonic means above the range of human hearing  of which the upper limit is around 20kHz) What frequency do they actually work at?
  3. What’s the build quality like? How long are they likely to function properly for?

Do they work?

Unfortunately the particular model purchased didn’t work for long enough (see below) to answer this question fully, but I’d say they did seem to make a difference initially. Further tests will have to be made on this front..

I took a look inside one to try and answer the second and third questions, see how it operates and whether any “improvements” might be made.

Here’s a picture of the various bits:


We have 2 x 1.2V 600mAh NiMH batteries, a solar panel, a buzzer of some sort and a little circuit board.

Here’s a close up of the circuit board:


Tracing out the circuit in LTSpice (an excellent free SPICE simulator) produced this schematic:

Mole Repeller Circuit

Note that 2N2222 and 2N907 are used in the SPICE circuit as LTSpice doesn’t have models for 9014 or S8550, which are the actual parts used on the PCB. This matters little to the simulation (see below) as the circuit should work fine with most “general purpose” bipolar transistors.

The function of the circuit is very simple – to briefly turn on the buzzer every 20 or so seconds. Here is a run through of the operation:

Q1 and Q3 are responsible for the on/off pulses. If we start with Q1 turned on (via current through R1 into base), Q3 turned off, and either side of C1 at around 0V. R7 gradually charges up the right side of C1 (Q3b) until the voltage reaches around 500mV at which point Q3 will begin to turn on. When Q3 starts to turn on, Q4 and Q2 (BUZ_V) will also begin to turn on, activating the buzzer. Also the bottom of C2 will be pulled to ground, which will also pull Q1s base (Q1b) low, turning it off – this will raise the voltage on the left (Q1c) and right side of C1, accelerating Q3s turn on. Things will stay like this for around 1 second until C2 charges up through R1 and the base of Q1 (Q1b) reaches ~500mV, at which point Q1 will turn on and  C1 will be pulled low, pulling the base of Q3 to ground and turning Q4 and Q2 (and the buzzer) off again. Then the process repeats….

If we run the simulation of the above circuit in LTSpice we get these waveforms (waveform names below correspond to bracketed names in the description and labels in the schematic above):

Mole Repeller Simulation 3

So, how accurate is the simulation? Pretty accurate as it turns out – here are the waveforms for Q1c (yellow) and Q3b (red) over 30 seconds, we can see they are almost identical to the simulation:


What frequency does the buzzer run at?

To answer this question and find out how the buzzer operates, I pulled the casing apart and took a look inside:


It’s a very simple construction, consisting of one resistor, a transistor of some sort, a magnet on an arm and a coil with two windings to produce what is called a “blocking oscillator”. The electromagnetic coil produces an oscillating magnetic field which makes the magnet arm vibrate against a diaphragm producing the sound.

Here is a schematic:


Some guesswork has been applied here as the (unmarked) transistor has 4 pins, not the usual 3. I think there may be a diode (represented by D2, possibly for temperature compensation) included in the package with it’s cathode connected to ground, so pin 4 would be the anode of D2.  The part numbers 1N4148 and BC847C are not the real part numbers, just a random diode and NPN from the LTSpice library.  The coil inductance values are guessed at too, but the resistor is definitely a 1.5k part.

This is not the most common blocking oscillator topology, with the control coil from base to ground (via biasing/compensating diode) rather than from supply to base (see below joule thief example) For further reading, Wiki has good pages on the blocking oscillator, and also the popular Joule Thief which utilises such an oscillator:

For the version I think is used in our buzzer I managed to track down a buzzer patent 4,065,733 which describes the use of a diode subject to the same thermal changes (e.g. ideally part of the same IC) as a compensating bias element.

The links above all have explanations of operation, but here is rough run through. If we begin with Q1 turned on, L_DRIVE_COIL begins to charge. Since it is coupled to L_CONTROL_COIL, the changing current induces a voltage across L_CONTROL_COIL keeping Q1 turned on. As the magnetising current slope levels out on L_DRIVE_COIL, the induced voltage into L_CONTROL_COIL drops. This lowers the voltage across L_CONTROL_COIL, stealing base current and begins to turn Q1 off. As Q1 turns off, L_DRIVE_COIL attempts to keep the current flowing and a voltage of the opposite polarity appears across it. Since it is coupled with L_CONTROL_COIL this accelerates the turn off process, with a voltage of the opposite polarity appearing across L_CONTROL_COIL bringing the base voltage below 0V. Q1 remains turned off until L_DRIVE_COIL discharges (i.e. “blocked”, hence the name) and the voltage on L_CONTROL_COIL drops, allowing the transistor to begin to turn on again.

To answer the frequency question, a quick test with the scope found out it runs at 359 Hz:


So, it’s not ultrasonic. My suspicion is that they all work around the same frequency (300-1000Hz) but some advertise as ultrasonic, presumably being unaware of the meaning (or maybe as they think it sounds better) Some ads (such as this one) advertise ultrasonic, but then state an operating frequency of 400-1000Hz in the specs, which seems to support this theory. The low frequency is apparently meant to simulate the sound of other moles burrowing. This makes more sense to me than an ultrasonic frequency for repelling moles, but I couldn’t find much data on what frequencies work best, or the hearing range of the mole.

What about the quality?

To put it simply, the quality of this product is pretty terrible. They are not built to withstand continuous operation outdoors. Reasonable proof of this was the fact that out of the 4 purchased, within a few months of use outside, all 4 were no longer working. So what went wrong?

Although there is nothing wrong with the principle of the buzzer operation, unfortunately the actual design of the buzzers in this product is not up to scratch. 3 out of the 4 failed due to the permanent magnet on the end of the armature coming loose as can be seen below (note the magnet is separated from the arm and lying on the white sealant stuff):


On inspection, it was found that the magnet is simply glued on with what appears to be something like super glue. This is simply not going to stand repeated vibration for long. Another issue is that the enclosure is not well sealed and none of the circuitry is “weatherproofed” (e.g. conformal coating or potting) This fact caused the remaining one to fail:



Next time we’ll have a look at designing a better version Winking smile


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  1. Gordon Gardner

    Hi, my names Gordon and i have been a proffessional mole catcher for over 20 years.
    Over the last several years i have been doing some research and experiments as regards talpa europaea and read your blog with interest.
    Can you tell me if, or have you found any scientific evidence as to what frequency a mole is adverse to. I have looked a great deal and find it difficult to find a precise frequency that a mole can actually hear.
    Best regards Gordon.

  2. Oli G

    Hi Gordon,
    Thanks for reading. I’m afraid I haven’t followed up the initial testing yet (partly due to other work and partly due to the moles in our garden ceasing hill building for a while) However, I intend to have another look at this in the near future (a couple of hills have appeared in the last few weeks) and will probably construct a few devices to test.
    From the small amount I have read, the moles can only hear low frequencies (see Wiki Mole page – Hearing ) so I intend to try a few different frequencies from 400Hz down to infrasound ranges (<20Hz) The last couple of sentences on the moles possibly having a direct pathway between the tympana is interesting, and if so would suggest they may be indeed sensitive to very low frequencies. I think the intention with current devices is to mimic burrowing by other moles, but have seen little evidence that it works.
    Anyway, hopefully I'll have some data soon which may be of some use to you, when I build the devices I have in mind. I will update the blog as I go along.
    Best Regards,

    1. lim

      Dear Sir,

      This is Lim of Korea Agriculture & Forestry System whise main items are all about pest control

      like birds, widldlife as well as insects

      Here attached with company brochure for your reference

      Anyway, I saw your mole repeller teardowm on your blog and impressed

      We are also now developing solar powered mole repeller using armorphous solar cell having better performance even at weak light

      also using micro buzzer on your blog and micro motor for vibration

      In this regard, can we ask for some technical advice?

      Here we can use high-end 3d print for prototype and shapable solar cell

  3. Steve

    Hi Oli G

    Likewise under occasional attack my moles in the garden, I too have tried a number of different acoustic mole deterrents on the market. Here are my observations:
    i) The principle of an acoustic deterrent is basically sound (pun intended!) – when the device is working properly.
    ii) They do not use ultrasound. Mole hearing does not extend to this region.
    iii) The deterrents create an intermittent pulse of sound, eg, a buzz for 0.5-1s repeated every minute. A longer duty cycle might be more effective, but would reduce battery life commensurately.
    iv) None of the deterrents I have tried have lasted longer than one season (3-6 months). They all have design flaws and a number of failure mechanisms ranging from water ingress and internal condensation, solar panel window clouding (UV exposure), mechanical wear (motor bearings and ‘clanger’ rods) to name a few. Solar power in the UK winter is, in my view, a forlorn hope – don’t waste your money. Buy a 4xD cell powered device and at least get a season out of it.

    I’m keen to develop a decent acoustic deterrent that will last years rather than months so am keen to hear from anyone with knowledge of mole physiology and psychology in order to be properly armed!

    1. Oli G

      Hi Steve,

      Thanks for the informative and interesting observations.
      You seem to have arrived at essentially the same conclusion as myself.
      I would be interested in developing a reliable deterrent device, and am quite sure it is possible if one does not try to build to what amounts
      to the absolute minimum in terms of material/component quality and design found in 99% of the devices found on the market at present.
      I have a few ideas that I would like to prototype and test, and although the electronics and design process is not an issue, I too would be interested in
      hearing from any mole experts out there who have some data that may be useful.

      Best Regards,

    2. lim

      Dear Sir,

      This is Lim of Korea Agriculture & Forestry System whise main items are all about pest control like birds, widldlife as well as insects

      We are also now developing solar powered mole repeller using armorphous solar cell having better performance even at weak light
      also using micro buzzer on your blog and micro motor for vibration
      In this regard, can we ask for some technical advice or cooperation?

      Here we can use high-end 3d print for prototype and shapable solar cell
      it would be also helpful to u also


  4. Gordon

    Ho Ollie and Steve Thanks for your replies.
    I have been unable to find any evidence of what frequency a mole is adverse to or make it react.
    I have conducted some experiments where i have placed several different makes of sonic vibrator each side of a mole trap. The mole has had to literally touch or brush past the sonic to enter the trap. I have yet to fail to catch a mole using this method.

    All the best. Gord.

  5. cb

    Interesting observations overall concerning the poor mechanical/functional design of these sound-based mole deterrents.

    I have tried a number of the basic “spike” type that use 3 of the D sized batteries, not solar. They do seem effective, at least for 3-6 months. Moles seem to acclimate to the sound though (same frequency buzz repeats at somewhat random intervals between 30-90 secs).

    Frustrated with the failure rate though, I set out to make a DYI adjustable sound generating device using parts from a dead spike- reusing the buzzer and housing). It runs at 5v (wired to maintain constant power) and emits a random clicking with ramping to simulate an animal version of a “DANGER” noise. Has been very effective so far. An Arduino was ($3 Arduino Pro mini) used for driving the buzzer and customizing the freq. and interval.

    1. Petar Petrovic

      Hi Cb on. Do you have link to schematics and sketch of your DIY Arduino mole repeller.
      I am very interested to build one myself.

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