Electronic pulse generators have many uses for both analog and digital circuits, and there are hundreds of different approaches to building a pulse generator. One of the simplest and least expensive has a total of three components: a 7400 NAND gate, a resistor and a capacitor. You wire two of the four NAND gates in the IC in a feedback loop and insert the resistor and capacitor in the loop to determine frequency. Once you have the parts and a power supply, you can put this circuit together in less than 15 minutes.
Things You'll Need
- 7400 quad NAND gate IC
- 7400 IC data sheet
- Prototyping breadboard
- Resistor assortment
- Capacitor assortment
- 5 V DC power supply
- Short pieces of 22-gauge wire
Read the data sheet for the 7400 integrated circuit (IC). Note that it’s a 14-pin device that takes its positive power supply at pin 14 and ground at pin 7.
Determine resistor and capacitor component values for the frequency at which you want your pulse generator to run. Frequency is given by the following formula:
f = 1 / (2.2 x R x C)
Where f is frequency in cycles per second, R is resistance in ohms, and C is capacitance in farads. For example, if you want a frequency of 50 kHz and you have a 1K ohm resistor handy, use a 9 nanofarad capacitor.
Insert a 7400 IC into the breadboard so that it straddles the channel or groove in the middle of the board. Take care so the pins go straight in and don’t bend or break. With the 5 V DC power supply off, connect positive power and ground to the breadboard. Using 22-gauge jumper wires, connect positive power and ground from the breadboard buss to pins 14 and 7 of the IC. Jumper pin 1 to 2, 3 to 4, and 4 to 5. Insert the resistor you selected in step 2 so one lead connects to pin 3 and the other connects to pin 1. Insert the capacitor from step 2 so one lead connects to pin 6 and the other connects to pin 1.
Turn the power supply on. Connect your oscilloscope ground clip to the power supply ground and the signal probe to pin 6 of the IC. You should see a square pulse wave at the frequency you used in step 2.
Tips & Warnings
- As frequencies go much above 100 kHz, the capacitance and inductance of the breadboard and wiring becomes a factor. Consider using a printed circuit board instead of a breadboard for higher-frequency pulse generators.
- Photo Credit ic, integrierter schaltkreis, integrated circuit image by Sascha Zlatkov from Fotolia.com
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