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PARTS LIST 1 PCB, code OE42F (from Oatley Electronics) 1 165mm length of 32mm 0. D. plastic conduit 2 end caps to suit conduit 1 momentary-contact pushbutton switch 1 battery holder (to suit 1 2V lighter battery) 1 12V lighter battery (VR22, EL 12, GP23 or equivalent) 1 piezoelectric disc transducer 1 12V piezo buzzer 6 PC stakes Semiconductors 1 2N5484 FET (must be specially selected - see text) 1 BC559 PNP transistor 1 4069 hex inverter IC 7 1N41 48 silicon diodes 1 5mm red LED Capacitors 6 1OµF 16VW LL electrolytics 2 0.1 µF monolithics 2 680pF ceramics Resistors (0.25W, 5%) 1 x 2.2M0, 3 x 1MO, 2 x 270k{l, 1 x 100k0, 3x3.3k0, 1 x4700, 1 x SOT (R6 - see text), 1 x 1 Mn trimpot Miscellaneous Threaded rod, nuts, hookup wire, solder etc. Where to buy the parts: a complete kit of parts for this project is available from Oatley Electronics, 5 Lansdowne Pde (PO Box 89), Oatley, NSW 2223. Telephone (02) 570 4985. The price is $24.95 plus $2.50 p&p. Note: copyright for the PCB artwork associated with this project is retained by Oatley Electronics. 30 SILICON CHIP FISH Bfl'f. DETECTOR Attach this simple unit to your fishing rod and you'll get an audible and visual warning when a fish is biting. This self-contained unit is easy to build and is simply secured to your fishing rod. It works by detecting rod vibration whenever a fish bites. No connection is made to the line and it has no on affect on normal rod and reel operations. When a fish bites, it flashes a LED and momentarily sounds a low-level piezo buzzer. As any fisherman knows, if you want to detect every fish bite you have to continuously hold the rod and "feel" the line. Alternatively, if the rod is placed in a rod holder which is embedded in sand, you have to continuously stare at the rod in order to detect a fish bite. This can become a little boring if the fish are only biting occasionally. The big advantage of the Fish Bite Detector is that it eliminates continuous "rod staring" or "rod R4 470{} D5 2x1N4148 0 B EOC GS 0 VIEWED FROM BELOW *SELECT ON TEST FISH BITE DETECTOR Fig.1: a piezoelectric transducer is used to detect vibrations and trigger the alarm. When vibrations are detected, Q1 and Q2 conduct, pin 6 of IC1c switches low, and the outputs of IC1d, e & f switch high to drive the LED and piezo buzzer. holding". And it solves the problem of using several rods at once, particularly at night. Some fishing reels are equipped with mechanisms that make a noise when a fish has taken the bait and is "on the run". However, these are generally quite insensitive and are only useful if the fish has well and truly hooked itself. By contrast, the project presented here is very sensitive and will respond to any decent nibble. And there's a sensitivity control that you can adjust to suit the size of the fish! Vibration detector As mentioned above, this unit works by detecting vibrations. More specifically, it detects rapid vibrations which are characteristic of a fish bite but does not respond to low frequency vibrations. This makes the unit insensitive to slow drags on the fishing line due to normal wave movements. To make the unit waterproof, the circuit is built into a 165mm x 32mm O.D. piece of plastic conduit which is fitted with end caps. The only external control is a pushbutton on/off switch which is fitted to one end. It alternately switches the power on or off with each successive operation. Power on/off indication is provided by the internal piezo buzzer. This emits a brief tone when the unit is turned on and gives a longer tone when the unit is switched off. Power consumption when the unit is switched off is negligible and is less than 2µ,A when the unit is on. This rises to a few milliamps when the unit is triggered, but this is for a short time only and has little effect on battery life. How it works Fig.1 shows the circuit details. It uses a piezoelectric transducer to detect vibrations and trigger the alarm. Let's take a broad look at the circuit operation first, before launching into a more detailed explanation. Alternate power on/off switching is provided by inverter stages ICla and ICl b, in conjunction with pushbutton switch S1. When the output of !Cl b is high, the amplifier consisting of stages Ql and QZ is on and it amplifies signals from the piezoelectric transducer. This stage then triggers a switching cir- cuit made up of gates IClc, ICld, ICle and IC1f, which in turn drive the piezo buzzer and the indicator LED. In greater detail, ICla and IClb are used to toggle the supply to Ql and QZ with alternate presses of S1. What happens is that the output (pin 4) of IClb simply assumes a logic level which is the inverse of that present on Cl when S1 is pressed. There's just one wrinkle here: there must be sufficient time (about 3 seconds) between presses of the pushbutton to allow Cl to charge or discharge via Rl. Let's say that initially pin 4 of IClb is low. This means that pin 1 of ICla will also be low, Cl will be discharged, and pins 2 and 4 will be high. If S1 is now briefly pressed, pin 3 will be pulled low and pins 4 and 1 will switch high. Pin 4 now supplies power to Ql and QZ via decoupling stage R4 (4700) and C4 (10µ,F). PIEZO TRANSDUCE / ~ Fig.2: you'll find the values for the various part numbers marked on the circuit diagram at the top of this page. Note that the resistors and diodes are all mounted end-on to conserve space. Use PC stakes to terminate external wiring connections. SEPTEMBER1988 31 The piezoelectric disc is made into a vibration detector by attaching a threaded rod and nut assembly close to the rim. The bottom rear edge of the disc is soldered to the earth track on the PCB. The circuit now latches in this condition and so Cl charges via Rl to the positive supply rail. If Sl is now pressed again, pin 3 of ICl b will be pulled low and thus pins 4 and 1 will switch low. Cl then discharges via Rl into pin 1 of ICla, ready for the next cycle. The piezoelectric disc is made into a vibration detector by attaching a threaded rod and nut assembly close to the rim. When vibration occurs, the piezo element is physically distorted and it generates an output voltage. This signal is then coupled via VRl to the gate of FET stage Ql. VRl is used to set the sensitivity while Dl and DZ prevent excessive voltage from being applied to the FET. The FET amplifier stage (Ql) is biased close to its cut-off point due to the high value of source resistance employed (R7 = lMD). Because FETs have such a wide spread in their parameters, a suitable FET and matching source resistor is selected on test (S.O.T.) and supplied as part of the kit. Transistor QZ forms an amplifier stage which is biased below cut-off by R6 (which is also specially selected). This means that QZ's collector normally sits at OV. However, if vibrations are detected, Ql and QZ conduct and QZ's collector rises to almost the full supply voltage. When this happens, the pin 6 output of IClc switches low and discharges capacitor CB (0. lµF) via diode D7. Thus, the outputs of paralleled inverter stages ICld, ICle and IClf switch high and drive the buzzer and LED indicator (the latter via current limiting resistor Rl2). Rl l and CB set the indicator time constant to around 0.2 seconds. As soon as vibrations cease, the output of IClc switches high again and CB charges via Rl l (2.ZMD). When the voltage on CB exceeds the inverter thresholds, their outputs switch low again and turn off the buzzer and LED indicator. Diodes D5 and D6 form an OR gate which isolates the indicator circuitry from the power supply switching circuitry. As we've already seen, the unit is turned on when S1 is pressed and pin 4 of ICl b switches high. To provide power on indication, this high is applied to a time constant circuit consisting of C3 and R5 to produce a 30ms pulse. This 30ms pulse is then applied to pin 5 of IClc via D5 but we still get a 0.2s indication period due to the Rl 1-CB time constant. Thus, the piezo buzzer sounds (and the LED lights) for 0.2 seconds whenever the unit is turned on. When the unit is turned off, pin 2 of ICla switches high and this signal is applied to a time constant circuit consisting of CZ and R3. This produces a 2.7-second pulse which is then coupled to pin 5 of IClc via D6. The piezo buzzer now sounds Keep all parts clear of the threaded rod and nut assembly to avoid upsetting the sensitivity of the piezoelectric transducer. We used a socket for IC1 but this can be considered optional. The piezo buzzer is connected to the PCB via PC stakes. 32 SILICON CHIP A SMALL PIECE OF FOAM RUBBER PREVENTS THE PIEZOELECTRIC DISC ASSEMBLY FROM TOUCHING THE PLASTIC AND STOPS MOVEMENT OF B SEVERAL SMALL HOLES ENABLE SOUND TO ESCAPE SHORT LENGT OF RIGID WIRE PREVENTS THE PIEZOELECTRIC DISC ASSEMBLY FROM TOUCHING THE SWITCH Testing Fig.3: how it all goes together. A small piece of foam rubber should be included as shown to stop the piezoelectric disc assembly from touching the case. The rigid wire stop prevents sideways movement of the board. Great care should be exercised when soldering the lead from the PCB to the front of the disc as it's easy to damage the thin metallic layer. Note the adjacent rigid wire stop soldered to the underside of the PCB. for the relatively long period of 2.7 seconds to tell you that the unit has turned off. Construction A kit of parts for this project is available from Oatley Electronics (see parts list). The kit is supplied complete and includes the printed circuit board (PCB), all on-board components, the piezo transducer, and the plastic conduit case. Fig.2 shows how the parts are mounted on the PCB. Install the parts as shown but leave the piezoelectric disc assembly off the board for the time being. Note that many of the parts are mounted endon to save space. Be sure to double-check the orientation of all polarised parts, as it's easy to make a mistake here. These parts include the diodes, transistors, IC, electrolytic cap- other end that goes to the disc. Note that the thin metallic layer lifts off easily with excessive heat. Do not try to tin this layer or apply heat directly with the soldering iron. The best procedure is to simply heat the back of the tinned lead with a soldering iron while holding it in contact with the disc. acitors, the LED, battery holder and the piezo buzzer. The piezo buzzer is supported on one end of the board by two PC stakes while two short pieces of tinned copper wire connect · the battery holder terminals to the PCB. Avoid using excessive heat and be careful not to bend the disc when working on the piezoelectric disc assembly. The first step is to solder the disc to the edge of the PCB as shown in one of the photographs. Tin both the edge of the disc and the relevant PCB track before soldering them together. The screw and nut can now be soldered to the top of the disc (see photo). Finally, a short length of hookup wire is used to connect the thin metallic layer on the disc to the input connection on the PCB. To do this, first connect one end of the lead to the PCB, then tin the Testing is no big deal. You simply set VRl (the sensitivity control) to its mid-setting and install the battery. If everything is OK, you will be greeted by a 2.7-second beep as the battery is installed. Pressing the pushbutton switch should now turn the unit on with an accompanying short beep. That same short beep should now occur each time vibration is detected. You will find that the unit is extremely sensitive just touching the PCB or the table on which the unit is resting should be enough to "set it off". VRl can now be set for the desired sensitivity. In practice, the sensitivity can be set quite high, because fish bites create a series of pulses that are quite distinct from false triggers. Final installation Fig.3 shows how the unit is installed in the plastic case. You will have to drill holes in the end caps to mount the LED and the pushbutton switch. In addition, a couple of small holes should be drilled adjacent to the LED to allow sound to escape from the piezo buzzer. Before sliding the assembly home, it is a good idea to slip a rubber band around the battery and the PCB to stop the battery from falling out of its holder. Also, a short length of rigid wire should be soldered to the earth pattern of the PCB (adjacent to the piezo transducer) so that the PCB assembly can not slide too far forward and foul the switch terminals. A couple of stout rubber bands or a hose clamp can be used to attach the Fish Bite Detector to your fishing rod. You will find that the unit will work quite well if attached about one-third of the way up the rod stem. ~ SEPTEMBER1988 33