Zener diode help

[joelap]

Hey guys, got my CE-3 working (the crackling sound went away after I replaced the DC adaptor... odd fix but it worked.)  The only issue I cant solve involves the LED.  In order to make the LED light properly on a 9V supply (CE-3 required the older 12V adaptor), a zener diode in series with the LED needs to be removed (D8).  That did the trick.  I replaced the LED for a high brightness clear LED, and because it is high brightness, the LED stays dimly lit when the pedal is off.  I tried upping R13 (significantly) and that only made the LED light dimmer when on (same intensity when it was supposed to be off).  Since I don't quite understand the concept of the electronic flip flop switching circuit, I tried changing the value of R12, but again made no difference.  Then I tried using a different Zener in place of the jumper in D8.  As it turns out, a 3V zener I had lying around makes the LED function perfectly.  However, after a little bit of working properly, the light started to pulse a bit.  And then stay off more than it would pulse quickly on.  I'm wondering if with the new 3V zener, I would need to increase the value of R13 to accommodate it?  Can anyone explain exactly what the Zener is doing in this situation?

The schematic, for reference:


Thanks,

Joe

[CGDARK]

The zener diode D8 and R13 acts as a battery check circuit and to prevent the LED to stay dimly lit as you stated. When the battery voltage drop to a certain level the LED will not light up so you know when is time to change the battery. So in order to make it function properly with 9V you have to carefully select the zener value and the value for R13 (try the values shown in the schem and modify as needed).

CG

[joelap]

Hi, thanks for that.  I've found a 3V zener stops it from staying dimly lit when off.  However, after a few seconds of being on, the LED turns off and doens't function anymore.  I'm guessing this means I should lower the value of the resistor, so more voltage is making it to the zener?  Or probably a better bet would be to have left the 5.6V zener in, and messed with the resistor.  :smack: Thanks.

EDIT: hehe... got 2 CE-3's here.  Took out the Zener on both and replaced the R's on both to 4.7k.  One is off when not active, the other is dimly lit!  Go figure!  Tried putting a 2k2 resistor on the other side of the LED, and it only made the same difference as if I upped the 4k7.  Also tried changing values of the R with the 5.6V Zener still in there, and none were making it light up bright enough, went all the way down to about 100 ohms, and then started getting worried about excess current draw so I stopped and bit the bullet.  I guess the simple solution would be not to use a high brightness LED in this situation... but who likes simple solutions anyways?

[Paul Perry (Frostwave)]

I bet that if you were to hang an analog (needle type) voltmeter off the battery, you would seethe voltage sag slowly after switching on. This would account for the led dimming and going dark.