NAND Gate Oscillator

[alchemist13]

Hi, I was just wondering, why is the nand gates (used as an ocillator) in the circuit below have its inputs shorted together? wouldn't they function as inverters? if so, why did the circuit maker use nand gates instead of NOT gates if he will make the nand gates function as inverters? Please reply.

 

[Harald Kapp]

Yes,
the NANDs here double as inverters. In fact you may use one, e.g. 4009.
Possibly NANDs have been used because there are often a few spare gatesleft when designing a circuit and NANDs are very versatile (as are NORs) because they can be operated as NAND, Inverter, or AND (NAND+Inverter).

Regards,
Harald

 

[alchemist13]

Thanks for the reply.

i have another question.

what if i use the circuit below

and remove the nand gate oscillators for the piezo buzzer so it would look like this

will there be any undesirable effect in the circuit aside from the fact that i have no piezo buzzer to produce an audible sound after the 4017 stops its inputs and stays in one LED?

 

[electro_pa]

The two invertor oscillator puzzle.

Hi alchemist13,
The NAND gate needs two inputs moving together to work as an inverter. The gate won't work if an input is left floating. IC2 a&b is working as two inverters.
If you need explanation of the two inverter oscillator, go to this link:
www.fairchildsemi.com/an/AN/AN-118.pdf
By the way the circuit has an error!
C1 is shown as 470pF, that is picofarads, but is drawn as an electrolytic capacitor. You can't have a 470 picofarad 'electrolytic!
It should be drawn as a plain capacitor, without the + sign.

 

[Harald Kapp]

yes and no

electro_pa is partly right: the unused gates of IC1 should be tied to VCC or GND via pull up or pull down resistors.
Otherwise, the circuit shown in your diagram 2 should work. If the switch/pushbutton is open, pin 8 of IC1 a is tied to GND and the oscillator will stop. If you push the start button, Pin 8 will go to High level and the NAND´gate will operate as inverter (you do not necessarily have to tie both input pins together - check the truth table. In the original circuit /1st post/ this is done only to avoid the necessity of a pull up resistor).

And yes, formally electro_pa is right with respect to C1 /1st post/, too. But I've seen that lapse many times before.


Regards,
Harald

 

[(*steve*)]

Your modified circuit looks fine. Aside from doing *something* with the otherwise unused inputs (as mentioned above), you should be fine.