0

I have a circuit with a ULN2803. The output pins of the ULN2803 are connected to 8 LEDs.

The problem is that irrespective of input voltage (either high or low) all output LEDs are glowing for a fraction of a second WHEN POWERING ON THE CIRCUIT.

After that only those LEDs are glowing whose input side has HIGH input, meaning it starts working normally.

enter image description here

ocrdu
  • 8,705
  • 21
  • 30
  • 42
smbabu991
  • 1
  • What are the inputs of the ULN2803 connected to? The issue is probably that the inputs are *floating*, could possibly be solved with pull-down resistors. – Klas-Kenny Mar 13 '22 at 18:21
  • A very warm welcome to the site. What is 'the circuit'? You're only showing a ULN2803, not what's driving it. Again, welcome. – TonyM Mar 13 '22 at 18:23
  • Can you give us a more complete schematic? What do you have COM (usually pin 10) connected to? You might try connecting that to the LED common connection (+9 V). With LEDs is should be okay to have it floating but I'd connect it anyway, it should not be tied to ground. – GodJihyo Mar 13 '22 at 18:26
  • Basically it is a water level indicator. All the inputs are connected with metal contact which are firmly fixed inside water tank. The inputs are 8 to 9 volt when water touches it. The inputs are not floating . Before this I used ULN2003 in which no sich issue was faced. – smbabu991 Mar 13 '22 at 18:40
  • Com is connected with a regulated power supply of 9 volt with help of 7809 and a 22 uf bypass capacitor across output of 7809. Input is also have a 2200 uf bypass capacitor – smbabu991 Mar 13 '22 at 18:42
  • @smbabu991 The ULN2003 and ULN2803 should both work the same. The internal circuits for the Darlington pairs are the same, the ULN2803 just has 8 of them instead of 7. Also you say the inputs are not floating, but the schematic and your description say they are. Pieces of metal connected to the inputs with a series resistor are floating unless they have some current path, if this is measuring water height they are floating until they come into contact with the water. – GodJihyo Mar 13 '22 at 19:03
  • @smbabu991 No you need to tell what is driving the ULN2803 inputs. If they are floating without nothing being connected, the leakage currents may keep the LEDs on. – Justme Mar 13 '22 at 19:05
  • @Justme you are right. Until water touches that level it need to be considered as floating. Then if leakage current is issue what to do next. – smbabu991 Mar 13 '22 at 19:11
  • @GodJihyo . you are right. Because of floating path now what to do next to solve this led flickering issue – smbabu991 Mar 13 '22 at 19:13
  • @smbabu991 I agree with others who say that this IC should behave identically. While the two datasheets offer some different numbers in their electrical specifications, the schematics are behaviorally identical, for all I can see. But I've a question. Is it your *only* problem that the ULN2803 powers up differently with a slight momentary "flash" that is basely perceptible? Once powered, the operation is identical for both? (I'm not sure I understand why that would matter to you, yet, if so.) – jonk Mar 13 '22 at 20:34
  • @jonk once powered UP it works fine without any issue like uln2003 one – smbabu991 Mar 13 '22 at 21:01
  • @smbabu991 Is this difference important to understand and fix? Or can you live with it? – jonk Mar 13 '22 at 21:54
  • @Klas-Kenny, just on pull-downs: like the ULN2003A, the ULN2803 has an internal pull-down series chain on each of its Darlington pair's inputs. It's about 13K so pretty solid. – TonyM Mar 14 '22 at 00:05
  • @jonk, no serious issue. can stay with it. – smbabu991 Mar 14 '22 at 03:29

0 Answers0