varied adc input to be displayed on a 7 segment display

[qwesy]

Hi,

I constructed an ADC (8bit) using LM339 and a variable resistor. I want to have a display on a 7segment using 7490ic and 7447ic by varying the variable resistor. Also, i would like to know or better if instead counting only upwards when the variable resistor is varied or both up and down.

Theoritically, the circuit should work but am having difficulty piecing the ADC to the "display end".

Please find attached.

[Trevor-B]

Hi,

I see you have one half of a 3-bit Flash ADC, a 555 and a counter that outputs to BCD connected to a BCD-7 Segment Decoder and the 7-segment display itself. Did your Professor ask you to use the 7490? It seems like the wrong chip for your application, unless you plan on using a multiplexer. I suggest using the Datasheet for the 7490 to make sure you are using the reset lines appropriately if you are planning on using it. 

[qwesy]

Hi,

Changed the 7490 to 74148 but the display doesn't change as the variable resistor is varied. "Individually" when the LM339 is separated from the circuit and connected to leds, they light up one after the other as the resistance is varied. 

Can you help me in pin pointing where i might have gone wrong?

Br

[Trevor-B]

Hi,

It looks like you are missing an opamp, as well as a ground connection for your resistor ladder. 

[qwesy]

Hi there,

The ground has been added, but not functioning as is supposed to. The objective is to get the 7segment display counting as the varaible resistor is varied. But once the circuit is powered it just reads "7" and doesnt move. I replaced the ADC part with normal switches and it functioned as it was supposed to .

Br,

Qwesy

[Trevor-B]

Hi,

As I said in my previous post, you are missing an opamp. When you used the switches, did you have 7 or 8? Did you have one input pin on you 74148 disconnected?

You need to be sure to mirror exactly the configuration you have for a comparison like that to be valid. 

[qwesy]

Thanks. Its functioning properly now.