555 Timer operating a PNP transistor

[mcghee.kevin]

Hi, just joined the forum. I am trying to build a timer, that after 5 minutes will turn on a buzzer.

My plan was to build a monstable 555 that when the output on pin 3 is high it would apply its voltage to the base of the pnp tranasistor which would turn the buzzer off, then when the 5 minutes are up (i have found the correct capacitor and resistor combination to do so) the output goes low and the pnp would have no power at the base and turn the buzzer back on.

I have both a functioning PNP circuit and a functioing monostable circuit but when i attach them together the 555 does not turn the pnp off... any ideas why, I am stumped.

[Audioguru]

The 555 has a darlington transistor output that has a high voltage that is 1.2V less than its supply voltage. The PNP transistor needs to have a base voltage that is at least 0.4V less than the supply voltage. Use a voltage divider to drive the base of the transistor or add two series diodes to its emitter.

A Cmos 555 like an LMC555, TLC555 and ICM7555 have outputs that go up to the supply voltage.

[mcghee.kevin]

thanks for the reply. First i would like to say that I am new to electronics although I learn quickly. My supply voltage is 9V and the output on the 555 is 6.4 volts if I remember correctly. I an unsure of what a voltage divider is or two series diodes to its emitter means... could you explain, perhaps post a schematic, I can understand a schematic very well.

[SgtWookie]

If you're only getting 6.4V out of your 555, there is a problem. Do you have a resistor between your 555 supply pin and the battery supply?

It would help if you posted a schematic of exactly what you have so far.

[Ron H]

Why don't you just connect the buzzer between +9V and the output of the 555?

[nomurphy]

If the buzzer draws more than ~200mA at 5V, don't connect it directly. Try using the discharge out (pin-7) as in the attached. Or, if discharge is being used for the timer, you can also do the same thing with the pin-3 output.

[mcghee.kevin]

Originally Posted by Ron H

Why don't you just connect the buzzer between +9V and the output of the 555?

ive tried that, when the output on pin 3 goes low and sinks to ground the buzzer comes on, when it goes high the buzzer is on but at a lower tone so that doesn't work

[mcghee.kevin]

Originally Posted by SgtWookie

If you're only getting 6.4V out of your 555, there is a problem. Do you have a resistor between your 555 supply pin and the battery supply?

It would help if you posted a schematic of exactly what you have so far.

No resistor between pin 8 and 9v+ I will be posting a schematic soon

[Ron H]

Originally Posted by mcghee.kevin

ive tried that, when the output on pin 3 goes low and sinks to ground the buzzer comes on, when it goes high the buzzer is on but at a lower tone so that doesn't work

So, do any of you guys understand how the 555 output can sink current when it's high? According to the schematic, it can't.
Kevin, are you using the same power supply for the 555 and the buzzer?

[JoeJester]

Ron,

The only way it could sink current when it's high is the potential difference between Vcc and whatever the other potential the buzzer is connected.

I'd wait for kevin's schematic before trying to decipher anything else.