TTL gates

[111]

I am building circuits using TTL gates and the outputs are observed on LEDs. But I get the output as only a small blinking of the LED for logic one. There is no malfunctioning with the power supply part as I used it as the supply for analog circuits. The regulated supply is taken from ac mains using 7805 IC. When the supply is given directly to the LED it blinks brightly indicating no problem with LED. I tried with different TTL Ics but the problem seems to be same.

 

[fsahmed]

Try connecting the LED as active low. If the problem persists, add a high value capacitor to the supply output (2200uf will do). If the problem doesnt persist, try using a transformer of greater VA value in the supply.

 

[audioguru]

No current limiting resistors in series with the LEDs! :shock: :shock:
111 probably thinks that LEDs are 5V low-current lightbulbs.

 

[111]

A high value capacitor is added and it now glows brightly for some seconds (till it discharges ) and again returns to low indication.

Hae guru there are current liming resistors in my work.

 

[fsahmed]

Where exactly have u put the capacitor? It was supposed to be between Vcc & ground, to clear up ripples. There shouldnt be any noticeable charging or discharging at all. Put the capacitor BEFORE the7805 as well.

 

[audioguru]

Hi 111,
What is the forward voltage of your LEDs?
How many hundreds of ohms are their current-limiting resistors?

Your 5V regulated supply shouldn't collapse with only a couple of 20mA LEDs connected to it. A 7805 can supply 1.5A if its input voltage is between 10V and 20V and it doesn't get too hot.
The absolute minimum input voltage for a well regulated 1A output from a 7805 is 8V if it is properly heatsinked. :lol:

 

[111]

I have connected the capacitor between the Vcc and the ground but i dont tried in connecting a shunt capacitor in front of 7805 as their is already a capacitor filter.

 

[Roff]

I t would be helpful if you could post a schematic of exactly what you are testing, including the transformer, rectifier, capacitor, regulator, etc.

 

[_3iMaJ]

The thing you'll notice about TTL chips (if you check the data sheets) is that they are terrible at sourcing current (1 = on), but they are great at sinking current (0 = on), so if you were to setup your logic as active low like suggested above and used your TTL chips as a current sink, and connect the anode of the LED to +5 volts and used a 330 ohm limiting resistor, you should see a nice bright light.