GB3MH Automatic Laughing Boxes

Renewed interest in the LPWS in 2011 has brought to light the story of the evolution of the GB3MH Laughing Boxes.

We have just been contacted by a long serving LPWS member who gave us 2 of the remaining boxes he built, and told us the history, many thanks!

Below is a mildly re-written version of the write-up he sent me, all I have done is correct spelling and grammar, no changes have been made to the details.

With all the other repeaters in the Midlands area well served with jammers etc, GB3MH remained fairly quiet, so in the late 1980's it was decided to give this languishing repeater a bit of a going over. GB3MH was in a prominent position on the Malvern Hills and, we believe, shared a mast with many other services including some operated by RRE Malvern. The hills are the highest point for miles around, and the tower's red light could easily be seen from over 50 miles away.

Operated by a bunch of loonies from Worcester, the repeater group were particularly badly served by GB3MH due to Worcester being at low altitude, when surrounding areas much further away, had a line of sight view of the tower and a much lower powered signal could blot out much more powerful stations in the Worcester area. The mast was not actually on top of the hill so coverage was very directional over Worcestershire. When driving just over the hill, the GB3MH signal would fade away and at about 1 mile down the other side of the hill, it could be barely heard.

Picture © Copyright Angus McCulloch and licensed for reuse under this Creative Commons Licence

First automatic prototypes were very crude by today's standards, but experience taught us that simple was better. We actually lost the first prototype, it was stolen by a group of amateurs from Worcester, but it was hardly hidden, just on test. A valuable lesson was learnt, so possibly money well spent, as we never lost another device after that one.

One of our very early boxes was accidentally remarkably successful. It did nothing much really, however the anger at what this simple device did, exceeded anyone's wildest dreams!

Nicknamed by the local hams "Tony Tone-Burst", it was originally deployed just to test the timer and tone-burst, and to see if it worked well out in the field, and if it would access GB3MH without the benefit of a proper antenna.

All it did was periodically transmit a 500 millisecond access tone-burst, but due to a complete fluke in the setting of the timing interval, it would resend the access tone-burst as the repeater was closing down and was sending the Morse ID. This meant that GB3MH was kept active with just a very short ½ second transmission burst, for reasons we can't imagine, no one would attempt to use the repeater when this device was operating.

There were at least 3 of these in smallish die cast alloy boxes, running on NiCads, one had a small coiled antenna plugged into a BNC connector, and 2 with ¼ wave wire and a crocodile clip. The first one could be deployed anywhere, the antenna could be removed and the unit plugged into a car or at someone's home.

The ones with crocodile clips would either run alone or attached to fencing or barbed wire somewhere in the countryside where the Malvern Hills could be seen, experimentation had shown that this worked well, even in the rain and some 30 miles away from the repeater. We assume that it transmitted using a kind of "long wire" antenna. It did work remarkably well, but we don't exactly know why.

A good feature of this simple device was that the batteries lasted almost a week, we tried leaving one hidden in Auction Rooms in Malvern, it ran 24 hours a day for over 6 days, and was recovered from its hiding place, on top of a cabinet, some 2 weeks later at the next auction.

Let's be honest about this, most radio amateurs are not "quick on the uptake", it was almost universally believed that this wasn't automatic, the loonies actually thought someone was sitting there pressing the tone-burst every 40 seconds or so, unbelievable!

A slight variation of this device was a simple timer circuit with a small relay built into a small plastic box on the end of a lead with a mike plug on the end. I saw this being used on an ICOM, the 2 sets of contacts on the tiny relay made a circuit between the pins for the PTT switch and the radio's built in tone-burst, the real beauty of this device was that 12 volts were available on the mike socket, so all you had to do was plug it in, no battery required. This was a novel adaptation, and cost less than £15.00 to make.

The One They Couldn't Find!

After these tests had shown what was possible, more elaborate devices were made. There were NO digital recording devices at the time, so the only useable audio source was an auto-reverse Walkman style AIWA cassette player. This worked, but the total power requirement was too great. After a meeting, and a "whip round" it was decided to make a more elaborate version of this device for a countryside location.

A Yagi 3 element beam was sprayed matt black, and with the help of a 3 section ladder, it was fixed way up in a tree, and pointed at the Malvern Mast. The box of electronics consisted of 2 chiming quartz clock mechanisms set 30 minutes apart to trigger the 30 second timer. The Tone-burst and transmitter boards were supplied by Wood & Douglas, both quite good and robust. The tone-burst automatically sounded when power was applied to the 1 Watt transmitter. Experimentation showed that hard wiring the audio from the Walkman to the mike input didn't work, it seemed to pick up too much local RF, so a small mike was fixed in front of the speaker of the Walkman, which was set to play auto-reverse with a tape of the laugh from the Laughing Policeman Song.

The antenna was connected to the transmitter and Walkman box which was also placed high up in the tree. The clever part of this set up was that a black cable was run down the tree trunk and attached with Evo-Stick, where it reached the ground, a shallow trench was dug and the cable went underground to emerge inside a pile of logs some distance away. Here, we were able to hide a car battery to remotely power the tree-top device.

All you had to do was clip the battery on, and off it went, twice an hour, automatically.

This was a quite expensive project, the transmitter and tone-burst cost about £100 from Wood & Douglas, the same for the Walkman, add to that a roll of black twin power cable, plus about £25 for a decent car battery, but it was highly successful, and even though it was D-Effed 100%, they couldn't find it! Their search was listened to on 70Cms, as they thought we were unlikely to hear them on that band. It is still funny today, to think back and to have seen all the undergrowth and grass round the tree flattened and downtrodden, but the device still operating. THE WANKERS COULDN'T FIND IT, despite being within 20m of it, what a bloody hoot! It's hard to imagine how they felt.

After the location was discovered, we didn't use it so much and it was eventually recovered and the parts used again in other devices although during this period we duplicated the effect from several people's homes in the area, just to keep them guessing.

Important Lesson

An important lesson was learnt here, you can't use a fixed location as someone will eventually find the device and steal it, so we embarked on designing a different type of device.

The main problems we came across with the prototypes were ingress of water and power supplies that would last a reasonable time.
There was no way we could find a useable remote mains power source, so we had to use battery power. At the time there were NO NiMh cells and no one had even thought of lithium ion, so lead acid jelly batteries from burglar alarms were used, luckily one of our members worked in this field and we were able to use nice compact 1.2Ah battery.

When we were making these, someone had a greeting card sent to them that had a small circuit and battery inside that laughed when you opened the card, brilliant, no need for the Walkman any more, £100 per unit saved. The audio output from the laughing chips worked straight into the mike connector. These cards cost around £5.00 each, they came with 1.5v button cells but these were replaced with Duracel AA cells, we never had to replace one.

 

Deployment

To confuse the opposition, which was 100% successful, we deployed 3 identical devices. Chiming quartz clock mechanisms were found to be very cheap timers, exactly the same as an ordinary clock movement but with 2 wires that momentarily completed a circuit once an hour. We set these 20 minutes apart so the 20-30 seconds of "the laugh" went off 3 times per hour, 24 hours per day. We found that this was more than enough to send the hard core loonies right round the twist!

Each box was in vastly different areas, but all with line of sight to the repeater mast, and each box had at least 2 locations. Each box was looked after by different people. It was their job to deploy the device, monitor the repeater for their box, which would be on the hour, 20 minutes past, and 20 minutes to. When the battery gave out, collect the box, recharge, and re-deploy in the alternate location. It sounds like a lot of effort, but it was really no trouble at all.

Experience dictated that this was the best way to achieve the most, with the minimum of effort, and with virtually no chance of anyone ever finding a transmitter box, unless it was by complete chance.

It was interesting listening to the DF crews on 70Cms, who had no idea what they were looking for, and no idea where they were they were hidden. It was a widely held belief that someone was driving about the area, that was their only conclusion when, after waiting another 20 minutes with the signal coming from a completely different direction.

 

As you can see from the pictures, they are dead crude, but they worked and they worked well, which is all that matters. As you can see, the main transmitter is secured by the power transistor heat sink, the tone-burst is also bolted to the casing. The laughing module is powered by an AA cell, and the 555 timer circuit is triggered by the pulse from chiming clock mechanisms. A waterproof power switch and an insulated tag for recharging complete the design, which was sealed into a die cast project box with silicone.

A slight problem we found with this design, was that the battery would give off a gas when recharging, that corroded the printed circuit of the transmitter board. The final version was much smaller with the battery outside the alloy case, this cured all the problems, but we were aware this could cause alarm if any of the units were found as it looked like a cartoon depiction of a time bomb. Luckily, LPWS newer had this problem, unlike the well reported copy-cat devices made in the Aylesbury area.