555 woes

[jimmythefool]

Hi all
I had a circuit that powered some IR leds directly, but redesigned it with the O/P (pin 3) going into the base of an npn transistor, via a 2k2 resistor. For some reason, the frequency 'dissapears' on the base end of the 2k2.. Any Ideas?

Thanks
Jim

 

[MoRoH]

I actually have the same circuit built, but i didn't check the base of the transistor, so i'm not sure what is on mine.

Do you have any output on the collector?

 

[GonzoEngineer]

Hi all
I had a circuit that powered some IR leds directly, but redesigned it with the O/P (pin 3) going into the base of an npn transistor, via a 2k2 resistor. For some reason, the frequency 'dissapears' on the base end of the 2k2.. Any Ideas?

Thanks
Jim

We aren't psychic.....can you post a schematic!

 

[jimmythefool]

Oops
I hope this has attached, and is readable


Thanks

Jim

 

[MoRoH]

there? yes
readable? no

aha try making it a bit bigger


-MoRoH

 

[jimmythefool]

hope this is readable..

Thanks Jim

 

[Boncuk]

The transistor will also saturate if you use 6.8K for a base resistor. May be that helps.

The "CV" input only requires 10nF.

Boncuk

 

[Roff]

Do you understand that the base voltage will only be ≈0.7V peak-to-peak?

 

[jimmythefool]

Thanks Boncuck, do you think a 2K2 is providing too much base current?
Most of the other designs I have seen, you an even smaller value.

I've got 0.5v across base and emitter Ron

Thanks

Jim

 

[Mikebits]

Thanks Boncuck, do you think a 2K2 is providing too much base current?
Most of the other designs I have seen, you an even smaller value.

I've got 0.5v across base and emitter Ron

Thanks

Jim

Are you checking this with a scope? I think going larger on the base resistor may be the wrong way to go, you are trying to drive a whole bunch of IR devices.

 

[jimmythefool]

I don't have one here Mike. I used a meter. I'm obviously doing something dumb, but cant see it!

 

[Mikebits]

Your meter probably will not do the trick. If I get your circuit, your trying to drive a bunch of IR leds off the collector circuit. Just as a test, try using a couple regular visable leds on say two jumper blocks, short out the remainder jumpers and see if that works, ie leds should light up. I could not read the part number of the transistor on your schematic so I hvae no idea of its beta.

 

[jimmythefool]

Thanks Mike
its a BC817-40 sot23

 

[jimmythefool]

Its fixed

The circuit works OK, the problem was in the batch of IR leds. The leads were cropped the wrong way round! (long lead on the cathode side)
Thanks for all the help though!

Jim

 

[Mikebits]

Good for you

 

[Roff]

Your meter probably will not do the trick. If I get your circuit, your trying to drive a bunch of IR leds off the collector circuit. Just as a test, try using a couple regular visable leds on say two jumper blocks, short out the remainder jumpers and see if that works, ie leds should light up. I could not read the part number of the transistor on your schematic so I hvae no idea of its beta.

Mike, did you consider that the LEDs or the transistor might be destroyed by running this test? If the transistor saturates (which it probably will), the collector (and LED) current could be over 200mA. If the transistor doesn't saturate, the power dissipation could destroy it, if the LEDs survive long enough.