Counting clicks (signals) at a radio frequency receoved by a shortwave radio

[Rob Rushworth]

Hello.
I'm an English language teacher researching the details of a class project; I use projects to develop language skills.
I have a background in mechanical engineering but know little to nothing of electrical engineering, hence this post.

Questions for members of this forum:

Electrical tennis racquet bug zappers... Do they discharge if they are not arcing?

1. If the operating switch is held down and no bugs cause an arc, will the battery of the device discharge at all? Put another way, if we tape the switch in the 'on' position but zap no mosquitoes, will we need to recharge the battery anytime soon?

2. Arcs from the tennis racquet that we have give an RF signal at around 3.8MHZ. This signal covers a distance of around 4 metres; zap a bug and hear a corresponding 'click' on your nearby shortwave radio.
Is it possible to build a simple device that can count the clicks?
Could anyone give directions to an instructional source or learning resource that would help us build such a device?

Many thanks for your help.

Rob

 

[CWatters]

They contain a circuit that steps up the battery voltage to a much higher value (2000V according to some web references). That circuit isn't 100% efficient, it uses power even when there is no output power (no arcing). So yes the battery will discharge when the button is held down and there is no arcing. Even when there is no arcing there might be some current leaking between the electrodes. Due to the voltage step up any small leakage current implies a much larger battery current. Many also have an LED on them.

In many countries buildings a spark transmitter/receiver set up could be illegal. Although enforcement might be unlikely unless you cause problems for other radio operators.

Yes it would be possible to design a circuit that plugged into the headphone socket of the shortwave radio and counted the clicks by I'm afraid I haven't time to design one for you. Basically it would probably need to rectify the sound and do peak detection, convert to digital, feed into a computer/counter/Raspberry Pi?

 

[CWatters]

It should be possible to measure the battery current and estimate battery life that way.

 

[Baluncore]

The arc discharge radiates energy at about 3.8 MHz. But the click is an audio envelope at audio frequencies. There may be several close pulses associated with one arc.

You will need to detect and rectify the click, then low-pass filter the detected envelope. That might be done with a crystal set, using the coil as the antenna. When first detected, add one to the count, then wait until the signal is gone, hold off for a set time to make sure the discharge has ended, then re-arm the detector and counter.

1. What is the maximum rate of clicks per second, or minute, that you need to count ?

2. How will you display the count ?

3. Have you considered building the counter onto the racquet ?
You could then avoid the radio link and detect the discharge internally, using the internal power supply for the counter and display.

 

[berkeman]

I'm an English language teacher researching the details of a class project;

hear a corresponding 'click' on your nearby shortwave radio.
Is it possible to build a simple device that can count the clicks?
Could anyone give directions to an instructional source or learning resource that would help us build such a device?

Have you done any Arduino or other microcontroller (uC) projects in your class yet? If so, it's straightforward to take the audio output of the shortwave receiver and put it into an analog input (or even a digital input) of the uC and write a small program to count the clicks.

If not, you should be able to find a small electronics project kit to do the counting. I'll do a quick Google search...

EDIT/ADD -- From a Google Images search:

https://www.google.com/search?q=ele...irzXAhVhxFQKHf0LBA0Q_AUICygC&biw=1241&bih=791

 

[Rob Rushworth]

Hello again.

Thanks for the replies - most helpful.

In sequence:

CWatters - so there is drain, I thought that there must be one somewhere... thanks for the confirmation.
The students found an Instructable lyesterday that shows how to permanently attach a racquet to a phone charger and keep it permanently charging / switched on, so we have solved that one. We'll be using ours to cap the entrance to a storm drain, as that's where many mosquitoes hide out in the daytime - we hope to zap them as they exit the drains at dusk; they're like bats in that regard.
http://www.instructables.com/id/Automatic-Mosquito-Killing-Bucket/
This 'racquet invention' is already a spark transmitter / spark-gap generator, so we won't be building an extra device. Its transmission range is around 4 meters as far as I can tell.
Good to know it's possible to count the sparks - we're on that one now.

Baluncore - 1. I have no idea about how many signals there will be or how often they will occur. We will be building a stationary and passive device to zap the mosquitoes as they leave the shelter of a storm drain at dusk. Yes, there will be several pulses / sparks / arcs, and we will be reducing the total count appropriately as and when we figure out the workflow of this idea. We can sometimes hear several arcs for one insect and think that perhaps there may be more pulses happening more quickly than our ears can distinguish separately, but that a counting device may register.
2. digital readout if possible
3. That idea hadn't occurred. Would it be simple?

berkeman - we have done no Arduino work or anything similar to it at all. In part, that's because I have no knowledge of it; my background is mechanical engineering.
Could I buy a frequency counter off-the-shelf that would work at the required frequency, or would I need to tune one or set one up?
Do you know of a forum, website, Facebook group, or otherwise that I could go to to find instructions on how to set up and use a frequency counter? I can hit Google for it, no worries, but if you know of a good one, that would help.

Thanks for the replies - we're moving ahead with this over the next 2 weeks or so.

 

[Rob Rushworth]

Ok, frequency counter sourced:
https://play.google.com/store/apps/details?id=com.keuwl.audiofrequencycounter&hl=en

initial settings - these give a single count for our audio signal > threshold level at minimum; noise level at maximum, gate time 1 second, measure = freq, direction = rise, coupling = DC, gain x2, time div = 320ms

The count from these settings is yet to be properly observed and confirmed, but they give a working start point.

 

[Baluncore]

I don't think a frequency counter will do the job you need done. You need to condition the detected pulses in the burst due to one insect and hold-off before the next. You need to count those insect encounters, without forgetting the count every second.

Ideally you might log the time of each insect encountered, then you will know their migration characteristics.

The zappers kill the fly, but they do not remove it from the mesh. That is one of the reasons why the zapper has a handle. How are you going to clear out the dead? Is the aim to count or to kill?

 

[Rob Rushworth]

We've tried the frequency counter app, and it counts single events while keeping a running total.
Starting with the tap of a pencil on a table, it can count the number of taps we make. Volume, ie a hard or soft tap, can affect the count - a hard tap can add more than one to the count, a gentle tap may not be recorded.
Now we're in the general ballpark re the device settings, we are trying counting with the radio and are simulating the clicks of zapped insects with a more easily found and predictable substitute - small leaves on branches of shrubbery.
Re conditioning the pulses, I think the radio may be doing that when we hear a click through the speaker; mechanical reproduction of the signal may be giving a smoothing effect - does that sound feasible?
Removal of the dead - not considered yet.
Logging zaps - that would be cool. I'll look into it.
The aim is to kill and count the number - this is a competitive event and the effectiveness of this device will be measured alongside others.

 

[Baluncore]

Re conditioning the pulses, I think the radio may be doing that when we hear a click through the speaker; mechanical reproduction of the signal may be giving a smoothing effect - does that sound feasible?

The waveform output by the radio will be limited by the bandwidth of the receiver audio. There will be a pulse for each RF input pulse, but they will be different shapes. I assume you are using an AM or a single sideband receiver.

By “conditioning the pulses” I mean detecting the first sub-pulse of a burst and setting a flag bit, then holding-off the reset of the flag bit until a set time after the last sub-pulse of that group is detected. That conditioning will turn the multiple sub-pulse bursts associated with each insect into single flag bit events that can be counted cleanly. The logic hardware required to do that is reasonably simple.