GF164 Nauman Gopher

Size: px

Start display at page:

Download "GF164 Nauman Gopher"

Ashlie Alice Harmon
5 years ago
Views:

1 GF164 Nauman Gopher Gadget Freak Case #164 Siren Short Circuits Gopher Invasion Ed Nauman owns a house situated just uphill from some city-owned property that provides his yard with an endless supply of pocket gophers. He has tried to defeat them from digging up his yard, but without much success until he tried a siren with several annoying tones. A simple timing circuit drives the siren with a low duty cycle. As Bill Murray said in Caddyshack, "In the immortal words of Jean Paul Sartre, 'Au revoir, gopher.'" Ed reported having tried dry ice, poison, smoke bombs, steel-toed boots, traps, and basically everything short of a thermonuclear device to stem the invasion. Even though many critters went to that great gopher city in the sky, their replacements arrived at the front within a week. "So I did some research into gopher hearing," said Ed. "Their hearing is reported as not a very broad spectrum but it has a pretty high gain between 800 and 2800 Hz." So with that information in hand, he waterproofed a 12-V, 125-dB siren that has several annoying tone sequences in the gophers' frequency range. Then Ed designed a simple 555-based timing circuit to cycle the speaker or and off with a low duty cycle and relatively short on periods to prevent the siren from burning out. He buried one speaker--sealed in a section of drain pipe--in the ground on the path the gophers use to tunnel into his yard. And he buried several more in the yard itself. Over the last two years, not a single gopher has shown up. (For package details, watch the video for this Gadget Freak case.) The popular 555 timer integrated circuit can serve many timing and pulse-generation functions. The timer relies on two resistors Ra and Rb and one capacitor, C, shown in the schematic diagram below.

2 GF164 Nauman Gopher Rev at 1145H 2 In this configuration, the circuit acts like an astable multivibrator, or oscillator. As explained in the book, "The 555 Timer Applications Sourcebook, with Experiments," by Howard M. Berlin, "... the timing capacitor C charges towards 2/3 V cc through R a and R b. When the capacitor voltage reaches 2/3 V cc, the upper [internal] comparator triggers the [internal] flip-flop and the capacitor starts to discharge towards ground through R b. When the discharge reaches 1/3 V cc, the lower comparator is triggered and a new cycle is started." For this circuit, you can calculate the charging period, t 1, with the equation: t 1 = * (R a + R b ) * C and the discharge period, t 2, with the equation: t 2 = * R b * C The complete cycle time, T = t 2 + t 1 or T = * (R a + 2R b ) * C where T represents seconds. You can calculate the frequency as the inverse of T, which yields:

3 GF164 Nauman Gopher f = 1/T = / ((R a + 2R b ) * C) In ED's circuit, however, the 1N914 small-signal diode "splits" the charge and discharge resistors. As a result, the diode bypasses the R2 resistor during capacitor charging, so the t 1 period depends mainly on current flow through R1. The discharge period depends on the circuit path through R2, as shown in the equations above. Then, in Ed's circuit, you can calculate the charging period, t 1, with the equation: t 1 = * R a * C (Note: R1 = R a ). and the discharge period, t 2, with the equation: t 2 = * R b * C (Note: R2=R b ) The complete cycle time, T = t 2 + t 1 or T = * (R a + R b ) * C where T represents seconds. You can calculate the frequency as the inverse of T, which yields: f = 1/T = / ((R a + R b ) * C) Find a complete LM555 timer datasheet from National Semiconductor at: <A HREF = " You can adjust the capacitor and resistor values to suit the on and off periods for a given siren, or you could create a second LM555 timer circuit to create a siren-like frequency. A Maxim MAX038 might make a better choice, though, for a sweep oscillator that could "move" its output frequency through a frequency band. For a data sheet, go to: Ed's circuit does not use this device. Builders can use an external power supply with an output of 16V to 22V DC to provide power to the LM340T-12 voltage regulator. The closer to 16V DC the better because higher voltages will create more heat from the regulator. The LM340T-12 can supply 1A of current at 12V. If you must drive a siren with more than 1A, use a separate 12V power supply and connect it to Pin 2 on the terminal block (TB1). DO NOT install an LM340T-12 regulator, but you may include C3 and C4 in the circuit. The MOSFET (Q1) can handle up to about 20A, but for more than 1A, attach the MOSFET to a heat sink.

4 GF164 Nauman Gopher Rev at 1145H 4 Ed created a small printed-circuit board in the format used by PCBexpress.com to produce prototype boards. A breadboard wired point to point would also work. You must supply an enclosure for the board and a 12-V power supply. Another option is to mount the circuit card inside the siren enclosure. The disadvantage is that you can t change the timing if it s sealed inside with the siren, but it does make for fewer wires to hook up in the field. I suggest using common hardware-store grade sprinkler-control wire or lowvoltage exterior-lighting wire to connect the low voltage side. This wire is made to be buried. Also, for the siren listed, your DC source should be capable of 1 Amp output. Happy gopher chasing. Schematic Diagram for the Gopher Chaser Bill of Materials for the Gopher Chaser Amt Part Description Source Part # 1 LM555 Timer IC, U1 Allied MΩ Resistor, 1/8W, R2 Allied Ω Resistor, 1/4W, R4 Allied Ω Resistor, 1/8W, R3 Allied

5 GF164 Nauman Gopher kω Resistor, 1/4W, R1 Allied μf Capacitor, 50V, C2, C4 Allied μf Capacitor, 25V, C3 Allied μf Capavitor, 50V, C1 Allied Contact Terminal Block, TB1 Allied Green LED, LED1 Allied IRFZ34NPBF MOSFET, Q1 Allied N914 Diode, D1 Allied LM340T-12, 12V Voltage Regulator, U2 Allied Siren, S1 Floyd Bell TXO W Floyd Bell: Allied Electronics: end-----

DEPARTMENT OF ELECTRICAL ENGINEERING LAB WORK EE301 ELECTRONIC CIRCUITS

DEPARTMENT OF ELECTRICAL ENGINEERING LAB WORK EE301 ELECTRONIC CIRCUITS EXPERIMENT : 4 TITLE : 555 TIMERS OUTCOME : Upon completion of this unit, the student should be able to: 1. gain experience with