[Moved]: Problem with 74ls148, voltage dropping below 5

[darkkey]

i am having a circuit that uses 74LS148 IC, input to the ic's are npn transistors and the circuit is powered by a transformer and a 7805 rectifier is in place. i have a problem, when i check the voltage in the circuit its around 2.5 to 3v. But if any one of the transistor (any one ) is turned on, then the voltage touches 5 and circuit starts working fine.

The ground and vcc to the ic is correct.

also what are the pins iam supposed to tie to ground ? i have E1 pin grounded ? Should i ground like GS or anyother ?

what might be wrong in my circuit ?..

I am having pull up resistors between transitor output and input to The encoder

 

[barry]

A schematic would sure help...

 

[darkkey]

A schematic would sure help...

I dont have one ready. Its simple Collector of every BC 547 is connected to ground. Emitter to 74LS148 as inputs.

 

[amayilsamy]

BC547 is NPN transistor , So How it is works when collector in ground?????

 

[ravindragudi]

I think you should include a bleeder resistor at 7805 output to draw some constant current. This will help regulator function as desired.

 

[darkkey]

Oops sorry. its the other way around.. Emitter to ground collector to Ic input. and their is 10k pull up betweem collector and ic

 

[amayilsamy]

Post your schematic. Orally we can't imagine your schematic.

 

[darkkey]

Power comes from 7805.

Input to base of transistor is from some sensors and other things

 

[Audioguru]

Your schematic shows that your 74LS148 has no +5V supply on pin 16 and has no ground on pin 8. All the pullup resistors also must be connected to +5V.

 

[betwixt]

Probably wants /GS grounding too.....

Brian.

 

[darkkey]

Probably wants /GS grounding too.....

Brian.

Well i thought it is an output pin ?? should i ground it ?

- - - Updated - - -

Your schematic shows that your 74LS148 has no +5V supply on pin 16 and has no ground on pin 8. All the pullup resistors also must be connected to +5V.

Brother, my circuit designer software hid them.. and i have mentioned in my post that ground and vcc are connected properly. also the 10k pull ups are connected to +5v itself

 

[Audioguru]

Do not connect the GS output to ground. It will go to ground by itself when it needs to be at ground.

 

[barry]

What's driving the base of your transistors? Don't just tell us "some sensors and other things". What kind of "other things"? Coat hangers? Eggplants?

 

[betwixt]

Tried Eggplants - they don't work. :-(

Apologies for the /GS mistake, I worked from memory and well... my memory didn't work.

Brian.

 

[Allen6502]

I simulated your circuit and it does work. I use 3 push buttons with a 10K resistor to represent your sensor.

Since this is a priority encoder. Only the highest number would be shown at the outputs. For example, if I press all 3 buttons at the same time the o/p would indicate '011' ie '3'.

I suspect your sensors are not giving enough bias to the transistors to make them conduct.

Allen