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• • • 1n1c This episode of Kit Clinic looks at the UHF remote switch as featured in our March 1988 issue. A lot of readers have had trouble with this project which will work only if you follow the instructions. Dear SILICON CHIP, I have built the UHF remote switch to team with my car burglar alarm. I urgently need to get it going because my neighbours are starting to gfve me a hard time. This is understandable because I've been fumbling for my car alarm switch at 6.00am in the morning and setting off the alarm. I've checked the unit out closely and so has a technician mate of mine and we can see nothing wrong with it. Can you suggest where the fault would lie? • One of the big problems about a circuit operating at 302MHz is that components are critical. Not only have you got to have the right components but you must mount them in the right way. Component lead lengths must be short. For capacitors, transistors and resistors and any other components for that matter, lead lengths should be no more than a few millimetres long. To demonstrate just how important it is to keep lead lengths short, consider the inductance of a lead only 10mm long and 0.5mm in diameter. This is given by the formula: L = .0021(2.303 log 1041/d - l)µH where 1 = length in cm d = diameter in cm For the above figures, this gives a result of 7 nanohenries. That can be very significant at 300MHz. It represents an impedance of 13.20. If you have a capacitor with leads 10mm long, the inductance of the leads can easily stop it functioning as a capacitor at 300MHz. IT WASN'T HARD TO SEE why the reader's transmitter (at left) wouldn't work. For starters, the .0022µF capacitor was much too large. But the real p~ohl_em was the BFR9~A transistor which was not properly connected into circmt. The photo at right shows how the transmitter should look. 78 SILICON CHIP Resistors are just as badly affected. If they have leads 10mm long their inductance and stray capacitance effects can play havoc with the circuit. All that is really just a preamble, setting the scene so to speak. The reader's problems When we had a look at this reader's transmitter and receiver units we had cause to wonder whether he had read the article or even looked at the accompanying photos. The transmitter had the most obvious problems. First, there was this humongous .0022µF capacitor which was much larger than in our prototype unit. The MC145026 integrated circuit was installed in a socket. This, combined with the fact that the capacitor was physically larger than it should have been, prevented the capacitor from laying over the top of the IC, as it did in our prototype. This meant that the reader had to resort to having quite long leads on the .0022µF capacitor and he had to sleeve the leads with cambric to stop them from shorting together. Well, IC sockets are more trouble than they're worth in a lot of circuits. We seldom use them in our projects because they are often unreliable. True, they do allow you to change ICs quickly but unless you do something silly like reverse the supply voltages, ICs are very reliable. In truth, neither the use of the socket or the large .0022µF cpacitor probably stopped the transmitter from working. We mention them solely because they were noticeable deviations from the original circuit and, as such, we would have expected a "technician" to fix them before giving up on the circuit. The real reason why the trans- A MUCH BETTER JOB had been made of the receiver although 5mm clearance holes had not been drilled to take the transistor bodies. It wouldn't work because lpF ceramic capacitors had been used where ever .0011,LF (1000pF) types had been specified. In addition, there were problems with L2. Once these problems had been cleared, the receiver worked as intended. mitter did not work was that the BFR91A transistor was not connected into circuit. Instead of being mounted on the underside of the board with its body sitting in a 5mm hole in the board, it was mounted on top of the board and with its collector and base leads soldered between the 2-6pF trimmer and 470pF capacitor, respectively. The emitter lead was soldered through to the centre hole which does not connect anywhere but was meant to be drilled out to 5mm in diameter. We corrected these deviations easily enough and after that the transmitter worked as it should. Another small point, which did not affect the transmitter operation, was that the LED leads were not long enough to allow the LED to protrude from the case. Receiver PCB On the receiver side, the problems were not so obvious. In this case, the two BFR91A transistors were at least properly connected into circuit although 5mm holes had not been drilled in the board to take the transistor bodies. A more serious problem was that 12 was not correctly wound and the coil had been glued to the coil former. Some of this glue had penetrated the threads of the slug and so it was glued solid and therefore not adjustable. That was enough to stop the receiver working properly but the real killer was the use of lpF capacitors where .OOlµF (l000pF) capacitors were specified. The three capacitors concerned are associated with the receiver input stage - there was no way it could work. And while we can well understand how easy is to be confused by capacitor labelling, the capacitors in question were simply labelled "1 ". Being very tiny ceramics, they could only be lpF and not .OOlµF. Having fixed 12 and replaced the lpF capacitors with .OOlµF units, the transmitter and receiver worked as they were supposed to. Lessons learnt It is our opinion that projects involving RF circuitry can be tricky for readers to build but if this project had been assembled correctly, so that it was identical to our prototype as shown in our photos, it would have worked. Note that no components had failed or been damaged. Nor had the circuit failed to work because some of the components had much longer leads than they should have had. We often see RF projects that have failed to work because of long component leads but in this case the problems were much more obvious. A careful visual inspection revealed all the faults. For our part, we are conscious that our assembly instructions need to be quite specific but if our instructions are ignored there are bound to be problems. ~ JANUARY 1989 79