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For dry beds and undisturbed sleep . . . Be instantly alerted when your toddler is about to wet the bed. This device will help train your child by immediate waking, before the bed is wet. Build this project and look forward to a happier child, dry beds and undisturbed sleep. Bed Wetting Alert by JOHN CLARKE F REQUENT BED-WETTING can be a real problem for many children. Occasional bed wetting by toddlers and young children is normal and can be tolerated but it can be a problem when it becomes a regular occurrence. Ultimately, it is in the best interest of the child to learn how to have a dry night’s sleep without the unpleasantness of waking up saturated and needing to have all the bedclothes changed. From the parents’ perspective, there is the constant work of extra washing of sheets, pyjamas and bedding after each bed-wetting incident has occurred. Commercial bed-wetting alarms are available but typically they involve the use of an aluminium foil under-sheet. They do work but the child and bedding still ends up being wringing wet 30  Silicon Chip and you still have to go through the process of changing wet bedclothes in the middle of the night. This is the big advantage of our new Bed-Wetting Alert. It detects the immediate presence of wetness in the child’s nappy and sounds the alarm to awaken the child who can then toddle off to the toilet. If not, the alarm will continue to sound at a much lower level, indicating to the parent there has been an alert condition which requires attention. The Bed-Wetting Alert comprises a key-fob sized transmitter and a receiver. The transmitter has an attached Velcro sensor strip with embedded wires to sense wetness in the child’s crotch and this causes the alarm to sound. The transmitter can be clipped to the hip section of the nappy or pyjama pants or placed inside the pocket of the pyjama pants. The hook side of the Velcro secures well to brushed cotton (flannelette) material and nappies. As soon as the sensor detects even a small amount of dampness, the transmitter sends a signal to the receiver to sound the alarm. The alarm sounds loudly for about 10 seconds before dropping in level. Should the alarm not wake the child, it will continue sounding at a low level until the sensor strap is dry or is removed, or the receiver is switched off. In order to be able to use a very compact transmitter module, we decided to base the circuit on Jaycar’s Secret Farter (Cat. GH-1088, $19.95). While this device has a trivial purpose, it is really a useful transmitter and receiver that sounds (!) when ever the transmitter button is pushed. As already noted, siliconchip.com.au Fig.1: this is the modified transmitter. It uses MOSFET Q4 to turn it on in response to low resistance (moisture) across the sensor inputs. the transmitter is housed in a small key fob style case, while the receiver is housed in a compact case with four AAA cells and a 50mm loudspeaker. The project assembly involves fitting a small PC board into the receiver case and replacing the transmitter’s PC board with a new unit which uses most of the transmitter parts. Transmitter circuit The circuit for the Bed-Wetting Alert transmitter is shown in Fig.1. It comprises a multivibrator using transistors Q1 & Q2 and an RF oscillator using transistor Q3 which operates at 304MHz. The circuit is powered from a 12V lighter battery. MOSFET Q4 is switched on when its gate voltage goes above about 3V. Normally, the gate is held at 0V by the 1MΩ resistor. However, when the sensor wires become wet, current flows from the 12V supply via the 100kΩ resistor, between the two sensor wires and via the second 100kΩ resistor and this pulls Q4’s gate high so that it can conduct and turn on the transmitter. Diode D2 is there to protect the gate siliconchip.com.au if the battery is inserted the wrong way around, while diode D1 protects the rest of the circuit if the battery is wrongly connected. The 10nF capacitor is included to prevent noise from switching on the MOSFET. When power is applied to the oscillator part of the circuit, the multivibrator starts and produces a 200Hz signal at the collector of transistor Q2. This waveform is used to turn on and off the RF oscillator comprising Q3, trimmer VC1, the L1 tuned circuit and the feedback applied to Q3’s base via the 2pF capacitor. The transmitted RF signal becomes bursts of 304MHz signal at a 200Hz rate. Bed-Wetting Alert circuit The receiver for the Bed-Wetting Alert is based on the Secret Farter. We will not describe its operation here but suffice to say that whenever the receiver detects the transmission signal, it produces a positive voltage to cause the on-board sound generator/ amplifier to drive the speaker. We use this positive voltage as the control input for the Bed-Wetting Alert Sounder The project uses Jaycar’s Secret Farter to obtain a low-cost transmitter and receiver. circuit which is shown in Fig.3. It is designed to produce a modulated tone when activated and this signal lasts for around 10 seconds before dropping to a lower sound level. The circuit is September 2004  31 Fig.2: this is the block diagram for the receiver circuit. It requires a few modifications, including the removal of the sound processor and amplifier (IC2), as shown here. based on a 4093 quad 2-input NAND gate Schmitt trigger package. Two of the NAND gates are connected as oscillators. IC1c is a tone oscillator operating at about 500Hz, as set by the 10nF capacitor, trimpot VR2 and the 100kΩ resistor from pin 10. IC1c does not oscillate continuously but is controlled by IC1a and IC1d, the gating oscillator which makes IC1c deliver a beep beep beep (about twice a second) instead of a continuous tone. Initially, when the control input is low (0V), transistor Q1 is off and the inputs to IC1a will be pulled high via the associated 1MΩ resistor. Pin 3 of IC1a is therefore low (0V) as is pin 12, so IC1d cannot oscillate. Because pin 12 of IC1d is held low, pin 11 stays high and it lets IC1c oscillate, which is not what we want. Therefore, D2 is connected between pin 8 and pin 3 so that when pin 3 is low, IC1c cannot oscillate either. When the receiver picks up a signal from the transmitter, the control input goes high, Q1 turns on, pin 3 of IC1a goes high and both oscillators start running. The gated tone oscillator signal is fed to the complementary buffer of transistors Q2 & Q3 and these drive the loudspeaker via a 22µF nonpolarised capacitor. Transistor Q4 provides a volume control for the Alert Sounder. When Q1 turns on to allow both oscillators to run it also pulls pins 5 & 6 low via the associated 10µF capacitor and so pin 4 of IC1b goes high. This turns on Fig.3: the Alert Sounder initially sounds the alarm for 10 seconds in response to the receiver’s control signal. It then drops to a lower level as Q4 is turned off. 32  Silicon Chip siliconchip.com.au Q4 so that it carries the full speaker current. After about 10 seconds, the 10µF capacitor is fully charged and so pins 5 & 6 are pulled high again via the associated 1MΩ resistor. Thus, Q4 turns off and the speaker is muted to a low level, by virtue of the much smaller current that now flows via its 1kΩ resistor to the +5.4V supply. The low-level sound continues until the power is switched off or the control input goes low. Construction As noted above, this project is based on Jaycar’s Secret Farter. The transmitter needs to be rebuilt onto a new PC board coded 15109041 and measuring 30 x 45mm. This board then fits into the original transmitter case and runs from the same 12V battery. A second PC board, coded 15109042 and measuring 57 x 42mm, is fitted into the receiver case behind the loudspeaker. Before starting, check that your Secret Farter works by inserting the batteries, switching on the receiver and pressing the transmitter switch to make a “socially inappropriate rude noise” – we can’t bring ourselves to say “fart” in this family magazine! If it doesn’t operate, check the batteries, as they may be flat. Having confirmed that the unit operates, construction can proceed. Start by checking the PC boards for any undrilled holes or breaks or shorted connections in the copper tracks. Fig.4 shows the component layouts for both the transmitter PC board and the Alert Sound board. Note that the hole on the transmitter PC board between the sensor connections and inductor L2 needs to be 3mm in diameter for the case securing screw. There are no mounting holes for the Alert Sounder PC board as it is held in position behind the loudspeaker once the case is assembled. Remove the parts one by one from the original transmitter and place them Fig.4: the new transmitter board (left) uses all the components from the Secret Farter transmitter board, except for the switch, LED and the 2.2kΩ resistor. At right is the parts layout for the Alert Sounder board. It uses low profile “skeleton” trimpots, while three capacitors need to be laid on their sides as shown in the photo below. The photo at right shows the completed transmitter board, while below is the modified receiver unit fitted with the Alert Sounder board. Table 1: Resistor Colour Codes o o o o o o siliconchip.com.au No.   3   4   3   2   1 Value 1MΩ 100kΩ 10kΩ 1kΩ 10Ω 4-Band Code (1%) brown black green brown brown black yellow brown brown black orange brown brown black red brown brown black black brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown brown black black red brown brown black black brown brown brown black black gold brown September 2004  33 transistors and capacitors. Note that the electrolytics must be oriented correctly except for the 22µF NP (nonpolarised) one. The three capacitors near IC1 also need to be laid over on their sides to allow clearance for the loudspeaker – see photo. Receiver modifications The sensor strap is made by threading two 0.2mm tinned copper wires (8A fuse wire) through the loops on a Velcro cable tie – see Fig.5. The wires are then fed through to the other side of the strap at the end with the pink loop. Each wire end is terminated to the shorting link in a jumper shunt. They are then connected via a 4-pin SIL header and 2-wire cable to the transmitter. This shows the completed moisture sensor and its companion transmitter unit. in the new PC board, making sure they are placed correctly and mounted as close to the board as possible, otherwise the lid will not fit. Note that there will be three parts left over: the switch, LED and 2.2kΩ resistor. Once all the original parts from the transmitter are placed on the new PC board, insert and solder in the remaining new parts. MOSFET Q4 also needs to be mounted squashed down on the PC board. For the Alert Sounder board, install the IC, resistors, diodes and trimpots first. Take care to orient the diodes and IC correctly. The specified trimpots are the skeleton type, not cermet. These are used to keep the component height as low as possible. The higher cermet type trimpots will not allow the case to be assembled since they will foul the loudspeaker. Next, install the PC stakes, the Before installing in the Alert Sounder board in the receiver, check that the new transmitter works. Do this by shorting the two sensor terminals with a wet finger or a screwdriver. In some cases, you may need to adjust trimmer capacitor VC1 ever so slightly to get the unit to transmit on the right frequency for the receiver. Do not move the trimmer by much before testing again. If the unit doesn’t work by turning the trimmer one way, try moving it the other way. Now remove the batteries from the Secret Farter receiver and then remove the PC board by taking out the two selftapping screws holding it to the rear of the battery holder. There are two ICs on this board, a dual in-line LM324 and a single in-line 6-pin IC which is the fart sound generator (IC2). This must be removed. Fig.6 shows how the board will look after modification. Remove the glue around IC2 and then desolder it from the PC board, as well as the loudspeaker wires. That done, replace C16 (4.7µF) with a 100µF 16V PC electrolytic. This capacitor needs to be laid over at about 45° to allow the case lid to fit. Make sure its polarity is correct, then solder a 100kΩ resistor across resistor R23. Next, attach wires to the GND and Control pads where the sound IC was mounted and to the +6V terminal on the slide switch. Pass the wires through the slot in the PC board. Reattach the PC board to the rear of the battery holder. Now place the Alert Sound board into the receiver case and wire up the GND, Control and +6V wires to Issues Getting Dog-Eared? Keep your copies safe, secure and always available with these handy binders. REAL VALUE AT $12.95 PLUS P & P Available Aust, only. Price: $A12.95 plus $7 p&p per order (includes GST). Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. 34  Silicon Chip siliconchip.com.au Parts List 1 transmitter PC board coded 15109041, 30 x 45mm 1 Alert Sounder PC board coded, 15109042, 57 x 42mm 1 Secret Farter (Jaycar GH1088) 1 pack Velcro cable ties 25 x 140mm (Jaycar HP-1210) 2 shorting plugs for 0.1-inch spaced header 1 4-way header plug 7 PC stakes 1 600mm length of light gauge figure-8 flexible wire 1 300mm length of 0.2mm tinned wire 1 100mm length of red light duty hookup wire 1 50mm length of white light duty hookup wire 1 50mm length of brown light duty hookup wire 1 100mm length of light duty figure-8 cable 1 10mm length of 3mm diameter heatshrink tubing 1 10mm length of 20mm diameter heatshrink tubing 1 100kΩ horizontal mount skeleton trimpot (VR1) 1 1MΩ horizontal mount skeleton trimpot (VR2) Fig.5: assemble the sensor strap as shown here. It is made from a Velcro cable tie, 0.2mm tinned copper wire (8A fuse wire), a 4-pin SIL header and two jumper shunts fitted with shorting links. the PC stakes. Also connect up the loudspeaker using the original speaker wires. Re-insert the batteries and check that the alarm works when the transmitter sensor contacts are shorted as before. If it doesn’t work, check the supply to IC1. There should be about 5.4V between pins 7 & 14. Trimpots VR1 and VR2 can then be adjusted to your preferred sound. Sensor strap The sensor strap is made using two Velcro cable ties (Jaycar Cat HP-1210), a 4-way pin header and two shorting siliconchip.com.au plugs. A short length of 2-way rainbow cable (eg, figure-8) connects between the transmitter sensor terminals to the header plugs. The shorting plugs act as connectors on the sensor strap assembly. Fig.5 shows the details. The Velcro cable ties consist of a strip of black Velcro with a square of yellow Velcro hook material at one end. The first job is to cut the yellow square off one strip and sew it onto the vacant end of another Velcro cable tie. The Velcro hooks will anchor well to the inside of flannelette pyjamas or a nappy. Semiconductors 1 4093 quad 2-input Schmitt NAND gate (IC1) 1 BC547 NPN transistor (Q1) 2 BC337 NPN transistors (Q2,Q4) 1 BC327 PNP transistor (Q3) 1 2N7000 N-channel MOSFET (Q4 on transmitter board) 1 1N4004 1A diode (D1) 2 1N4148 diodes (D2,D3) Capacitors 4 100µF 16V PC electrolytic (1 required for the Secret Farter receiver) 1 22µF NP or BP electrolytic 2 10µF 16V PC electrolytic 2 10nF ceramic (.01µF code 103 or 10n) Resistors 3 1MΩ 4 100kΩ (1 required for the Secret Farter receiver) 3 10kΩ 2 1kΩ 1 10Ω September 2004  35 Fig.6: this diagram shows the modifications required to the Secret Farter receiver board. Note that IC2 (the amplifier) must be removed. Fig.7: here are the actual size artworks for the two PC boards. Now sew two lengths of 0.2mm tinned copper wire (ie, 8A fuse wire) to the loop side of the Velcro strip and terminate each wire at the end with the material loop tag to a shorting plug. In each case, loop the wire through the side hole in the plug and around the exposed metal bridge. Note that the shorting plugs are to be located on the hook side of the Velcro and are secured to the Velcro using cotton thread. The two shorting plugs are placed side-by-side and sewn securely using the side hole to locate the piece and covering over with cotton to tie down the plugs. Since the Velcro cable straps come in packs of five, you can make two sensor straps, so that one can be washed while the other is being used. The 4-way header is used to make the connections to the figure-8 cable from the transmitter sensor terminals. The wires can be soldered as shown, with the whole assembly covered in some heatshrink tubing. The plug is passed through the material loop at the end of the Velcro strap and wrapped around this before being plugged into the two shorting plug sockets on the sensor. This will prevent the cord from pulling out. The wire length should be only as short as necessary to avoid it becoming tangled with the child’s limbs. The wiring into the transmitter should be protected with a sheath of heatshrink tubing at the point where the cable exits the case. You will need to file a small diameter hole in the base and lid of the transmitter case using a rat-tail file to allow clearance for the connecting cable. In use, the child should wear two pairs of pants or a pair of pants plus a nappy. The Velcro sensor strap is placed between the two pants, in the crotch area, with the hook side of the strap fastened to the nappy. You can locate the transmitter case either near th hip using the clip on the case or it can go in a pocket. The receiver should ideally be placed close by the bed so the child will hear the alarm. Of course, the whole procedure should be fully explained to the child, to help in the training process. Hopefully, after a couple of weeks, it can be dispensed with but if it has to be used for a long period, battery life in the transmitter should be quite good. However, the batteries in the receiver case may have to be replaced more often, as the alarm volume drops. The photo at left shows the modified Secret Farter receiver board, with the external wiring connections to the new Sounder Board. The photo at right shows how the +6V connection is made to the underside of this board. 36  Silicon Chip siliconchip.com.au