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Simple 9V battery + LED question

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ThePringler

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Hey guys,

I am building a little handheld digital temperature reader and display. I am using PIC to control it and a 9V Battery to power it.

My setup is simple. I have the 9V battery (positive) going through a switch and then going through a LM7805 voltage regulator. It then powers my PIC. I then have an LED tied to high with a 100 ohm resistor. My Problem is that the LED glows brightly but then quickly extinguishes (after about 15-20 seconds). My 9V Battery is brand new. Any ideas...?


Thanks!
 

Have you tried a larger resistor to reduce the current to the LED?
 

Have you tried a larger resistor to reduce the current to the LED?

Good call, ill give that a try. thanks

---------- Post added at 03:21 ---------- Previous post was at 02:44 ----------

Same problem with a 1K. Does a 5mm LED really draw all of the current out of a 9V battery?!
 

If Possible please add the drawing of you schematic .it will give better idea. As per my understanding you connected LM7805 out put to LED with series resister it right? .Then that case Led current is 5-2/100 = you are powering LED to 30mA.it is very less only .at the same time try to power up the PIC without connecting LED.To isolate the problem.and vice versa .Let me know it helped you.Please upload the circuit drawing for further understanding .
 

Same problem with a 1K. Does a 5mm LED really draw all of the current out of a 9V battery?!
PP3 9V batteries are rather puny but should definitely not be drained that quickly. You either have a bad battery or there's something wrong with your circuit or the actual components including the LED itself. An easy way to find out is to monitor the voltage of the battery with a multimeter starting from just before you switch the circuit on and while it's running.

There's one vital piece of info you haven't provided: Does the rest of the circuit continue to function after the LED is extinguished? If it does, then the problem must be with the LED itself.
 



---------- Post added at 03:21 ---------- Previous post was at 02:44 ----------

Same problem with a 1K. Does a 5mm LED really draw all of the current out of a 9V battery?!

9V battery is around 500mAHr so new one should put out 30mA for around 16 or 17 hrs. And I am sure that 9V battery can source more than 30mA. So no.
At 1kohm that would be 167hrs. But that is only the LED not the rest of the circuit. How much current is the pic draining?
Have you used a multimeter to see what is actually going on?
Debugging with an LED is not very effective, you need to measure actual voltages and currents to see why the circuit is failing.
Measure the actual 9v battery output.
Measure he 7805 output.
Measure the output of the PIC that the LED is connected to. Is that where the LED is connected to?
Measure the voltage across the resistor or LED.
Measure the current in to the PIC Vcc.
Measure the current out of the 9V battery.
After you measure then you can see easily what it happening.
I suggest you use a 9V regulator for the purpose of testing the circuit so you don't waste money on batteries.
Then when the circuit is working and you are confident that the circuit is not draining too much current, then try with the 9V battery. Unless of course you
are using rechargeables.
 

Which side of the 7805 is the LED wired? Is it across the switched 9v at the input to the regulator or is it across the regulated 5V output?

The value of the resistor should = voltage to be dropped / LED current. The voltage is what you start with, 5V or 9V depending on which side of the regulator you use, divided by the current you want to flow through the LED. As this is a power indicator and battery powered I would suggest the current is set to 5mA. Assuming a standard LED, the resistor should therefore be (9-1.6)/.005 = 1480 Ohms (use 1.5K) or (5-1.6)/.005 = 680 Ohms.

Next question: Is the LED getting hot? It shouldn't and if it is you have a wiring problem.
Same with the 7805, it should be running cold.

Brian.
 

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