I've connected a 556 as two one shots. Both circuits are identical. Threshold/discharge are connected to a 10k to Vcc and a 10u to ground. The trigger is connected to a 4k7 to vcc and the collector of a BC548 transistor. The output in both cases goes to a 7408. On one side, the output swings from a few millivolts to 3v3. On the other side the output swings from 1v5 to Vcc, I've removed everything from the outputs except the 7408. Why is it so? How can I make the 555 swing from a few millivolts up to Vcc?

 

There must be a fault somewhere. Either in your wiring or in the ICs themselves.

 

Please post the schematic and take the picture as 800x600 .jpg whatever you used breadboard or soldered on pcb.

 

You don't say what voltage VCC is. For the '7408 this must be 5V. Is the '555 supplied from this same 5V supply?

 

Welcome to AAC!

How can I make the 555 swing from a few millivolts up to Vcc?

You don't mention whether you're using bipolar or CMOS 555's, but you can't get what what you want in either case.

Bipolar specs:


Minimum VOL not specified, but typically 50mV. Guaranteed VOH 2.75V.

CMOS specs:



If you're driving TTL, why do you care what the output swing is? Both satisfy TTL logic levels:

 

Welcome to AAC!
You don't mention whether you're using bipolar or CMOS 555's, but you can't get what what you want in either case.

Bipolar specs:
View attachment 138974
View attachment 138975
Minimum VOL not specified, but typically 50mV. Guaranteed VOH 2.75V.

CMOS specs:
View attachment 138976
View attachment 138977

If you're driving TTL, why do you care what the output swing is? Both satisfy TTL logic levels:
View attachment 138978
View attachment 138979

Hi guys. Thanks for such fast and useful responses. I'm using an NE556N to drive an HD74LS08P, so I believe I need a voltage of less that 400 mV to secure a logic lo and more than 1.5 volts to secure a logic hi. I assume the 555 ought to be able to swing this?

 

I'm using an NE556N to drive an HD74LS08P, so I believe I need a voltage of less that 400 mV to secure a logic lo and more than 1.5 volts to secure a logic hi. I assume the 555 ought to be able to swing this?

More than adequate, but your voltages for 74LS08 aren't correct:

 

There must be a fault somewhere. Either in your wiring or in the ICs themselves.

Hi Marley
you have the right of it. I rebuilt the circuit and both ides of the timer are now working perfectly. Still can't find the fault on the board though, so I assume the chip was a dud.