Any obvious errors in this schematic?

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
I breadboarded the LED flasher circuit from page 8 of Mims Circuit Scrapbook, V. 2, as shown below; I used a new 9v Duracell. It worked, and I walked away for about 10 minutes, and when I returned, the LED was no longer flashing, so I put in another LED. Same result, but not believing my eyes, I tried a third LED and watched. It started off flashing at what looked to be a normal brightness, but quickly got dimmer and dimmer. Now, it's almost imperceptible. Admittedly, these are bulk Chinese LED's, but still...
 

Attachments

Jony130

Joined Feb 17, 2009
5,487
I breadboarded the LED flasher circuit from page 8 of Mims Circuit Scrapbook, V. 2, as shown below; I used a new 9v Duracell. It worked, and I walked away for about 10 minutes, and when I returned, the LED was no longer flashing, so I put in another LED. Same result, but not believing my eyes, I tried a third LED and watched. It started off flashing at what looked to be a normal brightness, but quickly got dimmer and dimmer. Now, it's almost imperceptible. Admittedly, these are bulk Chinese LED's, but still...
In this case LED is not guilty. You need add a resistor in series with the led.
 

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
In this case LED is not guilty. You need add a resistor in series with the led.
I was about to say the same, you can not run a LED without a resistor, ever. You burned yours out.
I wasn't blaming the LED's; I was blaming the schematic. I know that the current must be limited to an LED, but I thought that Mims knew more than I did. In fact, I actually did try the circuit with a 1 kΩ resistor in series with the LED. The LED lit, but it didn't flash. :confused: I had to drop the series resistor to 125 Ω in order for a green LED to flash. With a 9.4 V supply, that's more than 50 mA through the LED, albeit in very short pulses.
 
Last edited:

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
In the original circuit attached to post 1 of this thread, I can get it to oscillate if I change R1 to 10 Ω and R4 to 120 Ω. It's not terribly loud, and the waveform is quite ugly, but it oscillates and R5 does control the frequency.
 
Top