Benjamin C. Bisel
Figure 1 picture of circuit
The purpose of this circuit is to provide a timing device that can be used for detonating a bomb. It is a variable timer that can last from 6 to 55 seconds and provides 15V DC output for the detonation device, which may be altered to fit the consumers needs.
The circuit consists of four major parts. The first is a 555-timer set up in the mono-stable format (see Fundamentals of Electrical Engineering, Rizzioni p. 760) which outputs a square wave. To see the pin out diagram of the timer check outhttp://www.national.com/ds/LM/LM555.pdf This output is then hooked up to a 7493 four bit binary counter. This integrated circuit counts the number of pulses and has four outputs which all output high when the timer has output sixteen pulses. To see the pin out diagram look at http://www.fairchildsemi.com/ds/DM/DM74LS93.pdf These outputs are then placed into two and gates on a 7408 AND gate integrated circuit. The outputs of those two AND gates are then output into the last AND gate so when all four are high, the last AND gate also outputs high. To see the pin out diagram check out http://www.fairchildsemi.com/ds/DM/DM7408.pdfThis last output is then connected to the base of a 3904 transistor, which allows current to flow from the emitter to the collector and provides the 15V DC to the detonation mechanism.
The other parts of this circuit are the LED indicating how much time is left until detonation and the Potentiometer, which adjusts the time before detonation.
Figure2: Schematic of Circuit
The results of our testing this circuit showed that we needed to include a resistor between our third AND gate output and the base of the transistor. During our test, our AND gate stopped functioning properly, and that is a result of our having too much current drawn from it. In order to use the counter properly, the output of QA needs to be directly connected to CLKB. Another suggestion that is not included in this schematic is putting more LED's and resistors in series from the counter outputs to ground in order to see how much time is left before detonation. Also, for R4 the detonation system, we used a light bulb filament and black powder.