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We fitted our Battery Saver into a standard Eveready® Dolphin torch. These take an Eveready® No. 509 lantern battery but still have plenty of room inside. Simple battery saver circuit for torches How many times have you gone to use a torch only to find the battery flat because it had been inadvertently left on? Too many, right? Well, curse no more. This little project will save the life of your precious battery by turning it off when you’re not using it. Design by MARQUE CROZMAN The suggestion for this project initially came from a nurse who works at night. She was always coming across torches that had been left on or were flat as a result of being left on. In these times of being environmentally aware, she felt that wasting batteries needlessly just added to pollution, increased land-fill and so on, let alone the cost of having to replace them! The problem then was to come up with a project that turned the torch 24  Silicon Chip off when not in use, without being too expensive. In principle, the concept is simple enough and is the same as the “automatic power down” feature now present on many digital multi­meters and calculators. These turn the power off if the unit has not been used (ie, buttons pressed) with a given time, typically 15 minutes or so. Torch batteries have a much shorter life than the batteries in calculators or multimeters and they are usually used for shorter periods at a time. Therefore, we decided to come up with a circuit which would turn off the torch after a period of six minutes or so, unless it had been moved. So our circuit would have to have a timer which was reset each time the torch was moved. How to do that? Use a movement sensor, that’s how. If the torch is to be switched off, it stands to reason that any switching device used must have a negligible effect on the lamp brightness and it must also consume very little current in itself, whether the torch is on or powered down. That turns out to be a pretty stiff challenge. Read on to find out the solution. Circuit description The circuit is shown in Fig.1. When you turn the torch on, the circuit monitors its move­ment as you move around. When you put the torch down, there S1 TORCH 10M 470k 1M 0.1 MOVEMENT SENSOR 1M Q1 BC547 C B E 33 LL 2.2M TORCH BATTERY SAVER 8 4 RS Q2 7 DIS IC1 IRF540 7555 6 3 G THR OUT TRIG 2 1 6V 6V D S 0.1 B E C VIEWED FROM BELOW GD S Fig.1: the circuit is essentially a 7555 monostable timer with a period of six minutes (T = 1.1RC seconds). If the movement sensor ball makes & breaks contact, the 33µF capacitor is discharged & must charge to +4V before the torch is extinguished. ceases to be any movement. About six minutes later, the torch will turn off. IC1 is a CMOS 7555 timer which is configured as a mono­stable. The timing period is set by the 10MΩ resistor and 33µF low leakage capacitor at pins 6 & 7. When the torch is turn­ed on, the capacitor at pin 2 is initially discharged and this provides the trigger condition for the circuit. Pin 3 goes high and stays that way until the 33µF capacitor charges above +4V. When this occurs, pin 3 goes low. Pin 3 drives the gate of FET Q2 and this turns on to feed the torch globe. When pin 3 of IC1 goes low, Q2 turns off and extinguishes the torch globe. All this presupposes that the torch has not been moved after it was first turned on. If movement has occurred, the sequence of events is different. Any motion of the torch is monitored by the movement sensor. As shown in the photos, the movement sensor is TO-5 size metal can with eight pins around the periphery and one in the centre which connects to the can. Inside is a metal ball with a roughened surface. As the sensor is moved, the Fig.2: this component overlay diagram shows how to assemble the PC board. The movement sensor can be oriented in any direction. PARTS LIST 1 PC board, code 11101951, 30 x 35mm. 1 6V or higher voltage torch 1 movement sensor (available from Oatley Electronics) 1 piece of PC board, 17 x 7mm 1 3mm diameter x 12mm long screw & nut 1 33µF 16VW tantalum or LL electrolytic capacitor 2 0.1µF MKT polyester capacitors Semiconductors 1 TLC555C or 7555 CMOS timer (IC1) 1 IRF540 or BUZ71 N-channel Mosfet (Q2) 1 BC547 NPN transistor (Q1) Resistors (0.25W, 1%) 1 10MΩ 2 1MΩ 1 2.2MΩ 1 470kΩ metal ball makes and breaks contact between the can and one or two of the peripheral pins. Fig.3: here is an actual size artwork for the PC board. The heart of the Torch Battery Saver circuit is this movement detector. We’ve removed the top of one of these to show the roughened metal ball inside which makes & breaks contact between the case & any one or two of the outside pins. Note: photo is larger than life size. This intermittent contact charges and discharges the 0.1µF capacitor in series with the base of transistor Q1. So each time the ball inside the sensor makes and breaks contact, transistor Q1 discharges the 33µF capacitor at pins 6 & 7. This stops the 7555 from timing out and so the torch stays on. The Mosfet specified for Q1 is an IRF540 or a BUZ71. Both of these are cheap and readily available and more than capable of carrying the lamp current which will typically be about one amp or so for a large torch. The critical factors are the drain-source resistance of the FET and the gate voltage required to turn it fully one. In practice, these Mosfets require a gate voltage of at least 5V to get their drain-source resistance below 0.1Ω and thus reduce the voltage losses to below 0.1V. This makes the circuit practical only for torches with battery voltages of 6V and higher. It also means that once the battery voltage drops below, say, 4.75V, the losses across the Mosfet become quite significant. However, at 4.75V, a 6V torch battery has just about “had it” anyway. Table 1 sets out the operating conditions of the prototype Torch Battery Saver, as the battery voltage is reduced. As you can see, when the battery voltage is at 5V or more, the voltage losses across the Mosfet are quite low. Once the circuit goes into standby mode, the current is reduced to around 120µA which is mostly due to IC1. When the 7555 does time out and the torch turns off, the only way to turn it back on is by switching the torch off and on again. When you switch off, the triggering capacitor at January 1995  25 This view of the PC board shows the Mosfet bent upwards to reveal the 7555 timer IC. Note that our proto­type used a tantalum timing capacitor. This view of the PC board shows the Mosfet with its leads bent over & obscuring the 7555 timer underneath. pin 2 discharges through the 2.2MΩ resistor so that the 7555 can be triggered if you immediately switch on again. Construction We designed a small PC board for the Torch Battery Saver and it should be possible to install it in any of the larger torches. We installed it in an TABLE 1 Battery Voltage Current Drain Voltage Across FET 6V 750mA 0.061V 5.5V 720mA 0.068V 5.25V 700mA 0.074V 5V 680mA 0.086V 4.75V 670mA 0.114V 4.5V 640mA 0.200V 4.25V 540mA 1.032V 4V 370mA 2.610V 26  Silicon Chip Eveready® Dolphin torch and this had plenty of room inside. Fig.2 shows the parts layout on the board. Note that the IC must be installed before the Mosfet and the latter has its leads bent to lie over the IC. Mounting the unit in the Dolphin torch was relatively easy but we had to modify the central contact on the switch assembly. We did this by drilling a 3.5mm hole through the central contact and then made a new contact assembly which could be isolated from it. This was done by taking a small piece of copper PC board measuring 17 x 7mm. This had a hole drilled through the centre and a 3mm dia­ meter x 12mm long screw was soldered to the copper surface. This was then fitted with a transistor mounting bush and fitted to the central battery contact of the torch. The screw was fitted with a nut on the underside of the torch switch assembly and this then became the This photo shows how the central contact of the switch assembly was modified with a separate contact made from a piece of PC board. This was mounted with a 3mm screw (with its head buried in solder). This screw retains the small PC board which is on the underside of the switch assembly. negative supply contact for the battery saver PC board. The +6V supply to the board comes from the positive side of the switch assembly while the central contact to the torch bulb connects to the drain of Q1. Other torches will require different connection arrangements but we have designed the board with large positive and negative terminals to make this easy. Have a look at the photos to see how we did it. Note that when assembled, the retaining nut for the PC board will more than likely make contact with the case of the movement sensor. This is not a problem because the case is at 0V potential anyway. Testing The easiest way to test the device is rig it up to a 6V power supply or assemble it into your torch and turn it on. After six minutes or so, it should extinguish. On the other hand, if you move or shake the torch at least once every five minutes, the torch should not go out until you switch it off. Note that you can provide a longer timeout period by in­creasing the 33µF capacitor although for values larger than 100µF the leakage will become significant and ultimately will limit the period that can be achieved. You can also shorten the period, if you wish, by reducing the 33µF capacitor. For example, a value of 2.2µF will give a time of about 25 seconds. You could use a small value like this for testing, so that you do not have to wait SC out the full 6-minute period.