untitled

IC1is the other key component of this circuit. This is an LM555 timer

chip that you use to generate a square wave at its output on Pin 3.

R2, R3, R4, and C1 are three resistors and a capacitor, respectively, that

form the RC circuit that determines the frequency of the square wave

generated by the LM555 timer chip.

S1is an SPST (single-pole, single-throw; see Chapter 4) toggle switch

connected between the negative pole of the battery pack and the bread-

board ground bus. When this switch is open, no current can flow, and so

the circuit turns off. When this switch is closed, the circuit turns on.

LED2provides a light (we use a yellow light) to simulate a candle’s glow

in the pumpkin. This LED is on whenever S1 is closed.

R1is a resistor that limits the current running through LED1 to approxi-

mately 20 milliamps (mA).

R5 is a resistor that limits the current running through LED2 to approxi-

mately 30 milliamps (mA).

C2 is a capacitor that reduces the occurrence of noise on Pin 5, which

could cause false triggering of the IC. This might occur if Pin 5 were left

unconnected.

Now it’s time to run down the elements of the receiver schematic that goes

into the talking pumpkin. Take a look at the schematic in Figure 9-4.

1

6

7

84

5

2

R2

R3

C 2

C 1

IC 1

3

R5

+

• R4

LED2

S 1

6V

R1

LED1

Figure 9-3:

The silent

pumpkin

circuit

schematic.

190 Part III: Let There Be Light