I am working through through Electronic Circuits for the Evil Genius. One exercise uses two NAND gates to turn on an LED. (I don't have a good way to include an image of the circuit, so I will describe it as best I can.) The inputs to the first gate are connected to 9V through R1 and to ground through a push-button. (So inputs will be HI until NO switch is closed). Output from first gate is connected through diode to inputs of second NAND gate and resistor/capacitor pair. So, when input to first gate goes LO, output goes HI and Input to second NAND gate is held high by voltage in capacitor until enough voltage drains through R2 that input to second NAND gate goes LO. At this point, LED's neg. lead is connected through second NAND gate to ground. (LED's positive lead is connected to V+.)
OK...my problem. When I close the NO switch, the LED comes on, like it is supposed to, but as soon as I release the switch, it goes out. The book says it should stay on for about 8 seconds (the time it takes for the voltage to drain from the capacitor through the resistor to half of V+ at which point state of NAND gate changes.)
Since the LED stays off when contact is open and goes on when contact is closed, it seems like the basic circuit is breadboarded correctly. But since the LED goes off immediately, it seems like the RC timer isn't working correctly.
How can I debug this? It seem so simple, but I can't figure it out.
OK...my problem. When I close the NO switch, the LED comes on, like it is supposed to, but as soon as I release the switch, it goes out. The book says it should stay on for about 8 seconds (the time it takes for the voltage to drain from the capacitor through the resistor to half of V+ at which point state of NAND gate changes.)
Since the LED stays off when contact is open and goes on when contact is closed, it seems like the basic circuit is breadboarded correctly. But since the LED goes off immediately, it seems like the RC timer isn't working correctly.
How can I debug this? It seem so simple, but I can't figure it out.