How do you present electronic decision maker circuit?

Answer 1

There are two main parts to this project - an oscillator (based on IC1) and a LED driver (based on IC2). In a nutshell, when you press the pushbutton, power is supplied to the circuit and the "reservoir" capacitor on the main supply rail charges to the battery voltage (3V). At the same time, the resistor and capacitor around one of the Schmitt NAND gates (IC1c) cause it to oscillate. The word NAND is a contraction of NOT & AND. The "AND" part means that both inputs to the gate need to be a logic "high" for the gate to operate and the "NOT" means the output is opposite, or inverted, to the input. There is a "truth table" shown in Table 1 which shows what happens to the output, depending on what is occurring at the input. The "Schmitt" part of the name refers to a feature of the threshold points, or triggering, of the gate. The voltage levels at which it triggers, either low or high, are quite precise but more importantly, are widely separated. This makes a Schmitt trigger more immune to noisy triggering waveforms. As you can see, the two inputs to the gate are connected together, effectively turning it into an inverter. As such, the input and output can never be the same state - when the input is high, the output must be low and vice versa.

When you press and hold the button, the IC is powered up but at that instant the inputs are in a low state (because the 1μF capacitor is not charged). Therefore the output is high. The capacitor then starts to charge via the 68kΩ resistor from output to input. When the capacitor voltage passes the gate's upper threshold voltage (ie, the input goes high), the output goes low. The capacitor then starts to discharge, the voltage eventually dropping below the gate's lower threshold voltage. The output then goes high again. This keeps happening as long as power is applied to the circuit. It's called a "relaxation oscillator" and is a very easy way to make any form of pulse generator. The frequency at which it operates is determined by the values of the resistor and capacitor. The formula is 1/0.55 x RC, where R is in ohms and C is in Farads (note that - Farads, not microfarads). Therefore if the resistor is exactly 68,000 ohms (unlikely!) and the capacitor is exactly 1μF (even more unlikely!), the frequency of this oscillator circuit will be 1/ 0.55 x 68,000 x 0.000001, or 1/0.0374, or approximately 26Hz (actually 26.7Hz). Why did we say it was unlikely that the resistor and capacitor wouldn't be exactly what their marked value said? If the resistor has a 1% tolerance, its actual value could be anywhere from 99% of 68,000 ohms (67,320Ω) to 101% (68,680Ω). And capacitors normally have a much wider tolerance - as much as 20% or more. So you can see we are not talking exact values in a simple circuit such as this.