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Salvage It! BY JULIAN EDGAR An automatic stopwatch timer Do you need to automatically record equipment running time? This stopwatch timer starts when power is applied to a low-voltage (12V) circuit and automatically stops when the power is switched off. When power is again applied, the stopwatch continues to count from where it last stopped. T HE AUTOMATIC stopwatch timer is a cinch to build and resetting the count is as easy as pushing a button. And by using a snap-action thermostat (salvaged, of course!), it’s even possible to automatically measure how long a temperature is above or below a set-point – ideal for environmental, solar heating and machinery monitoring. Because it uses salvaged parts, the complete project should cost you only a few dollars. Let’s see what’s involved. The components You’ll need an old digital watch with a stopwatch function, two relays (one a double-throw design), a capacitor and a pushbutton switch. Oh yes, and something to mount the components on! We used a small piece of pre-punched board laminate (unclad) and that was literally the only component that we had to buy new! It’s quite likely that you already have a digital stopwatch tucked away in a drawer – as fashions change, lots of people buy new watches even when the old one is still working fine. If you have to buy a new one, shop around at discount stores – it shouldn’t be too hard to find a watch that includes a stopwatch function for about $5. The unit shown here came from a watch I’d not worn in years – in fact, until I went looking, I’d forgotten I even had it! Relays can be found in a huge amount of discarded electrical equipment. The ones used here are quality Omron designs that were salvaged from an old radio transmitter that had been sent to the tip. Although they use 24V coils, they work fine in this application which is based around a 12V supply. Note that if you want to use the timer to monitor even lower voltage equipment, you’ll need lower voltage relays; eg, 5V. Don’t use this project to monitor voltage rails above about 15V DC, otherwise you will damage the circuit components. What about the 220mF 25V electrolytic capacitor used here? Well, again they’re everywhere in older electrical equipment – just take a look! Finally, the monetary pushbutton switch was salvaged from behind the front control panel of a VCR. All older VCRs with click-action pushbutton controls have this type of switch buried behind the faceplate. How it works This digital stopwatch timer will automatically measure how long low voltage equipment is running, making a cumulative count until reset. . Most of the parts can be salvaged for nothing. 46  Silicon Chip Fig.1 shows the wiring diagram (the tricky circuit was designed by John Clarke). Let’s look at Relay 1 first – its coil is wired in parallel with whatever device we’re monitoring – so when the device is on, so is Relay 1. This closes the Normally Open (NO) and Common (C) connection, feeding power to the coil of Relay 2. The 220mF capacitor is wired in series with Relay 2’s coil, so Relay 2 will pull-in only for as long as it takes the capacitor to charge. In this case, using the relays and capacitor specified, that takes about 0.1s. Relay 2’s NO and C connections are wired to the contacts that normally start and stop the stopwatch. siliconchip.com.au you work out which are the Normally Open (NO), Normally Closed (NC) and Common (C) relay contacts. Often there is a small diagram on the relay itself, or you can use the continuity function of a multimeter to check the contact behaviour. Take care when wiring in the capacitor, as this device is polarised. Its negative side is shown by a line of “-” symbols near one of its leads. Fig.1: two relays, a capacitor and an old digital watch are used. The circuit is configured so that the start/stop button contacts of the watch are momentarily connected by Relay 2 when power is applied and the same occurs when power is removed. So when power is applied to Relay 1’s coil, Relay 2 is momentarily pulled in and the stopwatch is triggered. That’s the starting pulse – now what about the stopping pulse? When power is removed from Relay 1’s coil (ie, the monitored device is switched off), Relay 1’s NC (normally closed) and C terminals are connected. This immediately provides a path for the capacitor to discharge through the coil of Relay 2, so again momentarily pulling it in. Bingo! – we now have a switch-off pulse. The reset button simply bridges the watch’s original reset contacts. Building it Using It Fig.2: here’s how to use the stopwatch timer to detect how long a temperature is below a setpoint. Thermostats found in much discarded equipment (eg oil-filled electric heaters) open when the temperature rises above a setpoint. Wired as shown here, the count will stop when this occurs. The first step is to remove the rear of the watch, carefully pull out the workings and inspect the start/stop and reset buttons. Normally, a single common is connected to a PC-board pad to start and stop the count, and to another pad to reset the timer. You need to solder a wire to the common and then two others to the start/stop and reset pads. As might be expected, it’s easier to do this in some watches than others. Note too, that in some watches, the common comprises a “springy” stainless steel strip which is impossible to solder. If this is the case, bend the strip over and crimp it to the wire. Once you have the three wires coming from the watch, check that you can start, stop and reset the stopwatch. siliconchip.com.au After that, it’s just a case of following the circuit diagram. Make sure that Testing the unit is easy. First, connect the +12V and earth leads to the power supply, then connect the 12V trigger wire to +12V. The timer should start running and continue for as long as this wire is connected to the +12V rail. Now disconnect this wire – the timer should immediately stop. Finally, press the reset button and – well, you can guess what should happen! Note that if you just brush the trigger wire against the +12V rail, the relay contacts may “bounce”. This can result in the timer getting out of sequence – ie, running when it should be stopped and stopped when it should be running. If this happens, manually bridge the start/stop terminals to return it to the correct operating sequence. If you find that both relays click but the device isn’t working as it should, try increasing the value of the capacitor. If the relays have clear covers, it’s interesting to watch Relay 2 quickly pulsing when the watch starts and stops. When running, the prototype draws about 50 milliamps (50mA) so if you are monitoring a battery-operated device, this should be considered. SC Any old digital watch that has a stopwatch function is potentially suitable for this project. However, the ease with which wires can be soldered to the switch pads varies from watch to watch. When selecting the watch you should also take note of the maximum time the stopwatch display can show. May 2005  47