CMOS 555 Output Voltage drop

Hi all. I am using an LMC555 for a monostable circuit in a LV project. The chip is set up to operate a buzzer via pin 3 every time I trigger it. I have been forced to use a transistor to drive the buzzer because the voltage level at the output pin is too low. The supply voltage is about 3.2V (2 X AAA) and the datasheet specifies only a small voltage drop at the output. My buzzer can sound at 1V but when I connect it between pin-3 and Ground I get nothing. Measuring the voltage across it when the LMC555 is triggered I get about 1.5V. If I replace the CMOS 555 with a normal 555 timer (~NE555) the circuit works fine. I don't want to use a normal 555 timer though because it won't work when the batteries fall below 3V, draws too much current, and requires two more capacitors in my circuit.

So my issues are: - Why am I getting such a small output voltage in comparison to the supply?
- Why isn't my buzzer working with the LMC555 even though it can sound at 1.5V?

Thanks, Trevor G.

Your buzzer is probably loading down the timer's output. Using a transistor to buffer the signal is correct.

Your battery is 3.2V when it is brand new then its voltage quickly drops to 2.4V.

The datasheet of the Intersil ICM7555 shows a graph of its typical output current and voltage when the supply is 2.0V, 5.0V and 18.0V.
With a 2.0V supply its output high current is only 1mA when it has a 1V loss. Its current is only 1.1mA into a dead short.
With a 5.0V supply its output high current is only 5mA when it has a 2.5V loss. Its current is 7mA into a dead short.

The output low current of a Cmos 555 is much higher.

The output high or low current of a normal 555 exceeds 100mA when it has a 2V loss, a huge difference.

Your buzzer probably needs a low driving impedance but a Cmos 555 has a fairly high output impedance.

I knew you'd come up with the numbers and a graph. I tried to beat you to it, but couldn't find the info.

BrownOut said:

I knew you'd come up with the numbers and a graph. I tried to beat you to it, but couldn't find the info.

Click to expand...

That's funny. I usually hope someone else will find the data, so I won't have to.

I didn't bother posting the graph because many people do not understand that the Cmos 555 has a fairly high output resistance (which results in a voltage drop and low output current) when its output is high.

I guess this would be the disadvantage of the CMOS 555 over the traditional 555. My buzzer draws about 1.5mA at 3V, I expected the chip to be able to handle that much at least. I was going off the National LMC7555 datasheet and it states that the chup can source up to 50mA no worries and at Vs = 5V and Isrc = 2mA, the typical output voltage is 4.7V. Looks like I'll have to stick with the transistor man for my little buzz.

No. The datasheet says Voh is 4.7V (typ) at only -2mA when Vs is 5V.

Oh wait, you are using a different part. You said LMC555.

trevor261 said:

I guess this would be the disadvantage of the CMOS 555 over the traditional 555. My buzzer draws about 1.5mA at 3V, I expected the chip to be able to handle that much at least. I was going off the National LMC7555 datasheet and it states that the chup can source up to 50mA no worries and at Vs = 5V and Isrc = 2mA, the typical output voltage is 4.7V. Looks like I'll have to stick with the transistor man for my little buzz.

Click to expand...

There is an LMC555, a TLC555 and an ICM7555 and they are all Cmos 555s with the same spec's.
National's datasheet says that it "is tested to -10mA and +50ma output levels" but the spec's show that its output high current is 10mA when its supply is 12V and its output voltage loss is a max of 1.5V. Its output low current is 50mA when its supply is 12V and its output voltage loss is a max of 2V.

I suspect that your "buzzer' is actually a piezo beeper with a built-in oscillator that uses 1.5mA at 3V but needs to be driven from a low impedance. The LMC555 will have an output of 1.5mA at 3V when its supply is about 4.5V. Try it with a 100uF capacitor on its output to ground to make it a low impedance.

They show much less current at lower supply voltages.