# 555 To Drive Circuit? Possible or Not?

#### paw

Joined Jan 11, 2012
18
Hi All,

First of all I want to start by thanking you all in advance for replies!

I have a circuit using a 555 setup in a monostable configuration. Now, this is set up so that the output voltage goes high when the power to the circuit is switched on.

It then remains high for 10 seconds, and then goes low.

Now, I have tested the output with a resistor LED combination, to make my LED light up after the 10 second period.

I now want to power a whole (simple) circuit, rather than just an LED.

The diagram shows how my simple circuit is setup. Note that the input voltage labelled 12V varies between 0V and 12V to alternate which LED is actually on. This input voltage will come from an external sensor.

I then though that I could power this circuit via the output of the 555 Timer, i.e. due to the fact that the output of the timer goes low after 10 seconds, connect this output to where on the image it shows the transistors are grounded. Now, connected the +V rail to +Volts would therefore mean that the circuit would only switch on when the output of the 555 is low because of the fact that when the output is high, there would be a potential of approximately 0V across the emitter and collector of the transistors.

I am not really sure if my way of thinking is correct, and I would appreciate your guidance and advice!

Thanks again,
Regards,

Paw

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#### Wendy

Joined Mar 24, 2008
22,932
Depends on the current you need, which you did not mention. There is also the fact that a 555 outputs about 1.3V less than the full power supply voltage.

You could attach your picture to the thread (most people are leary of clicking an unknown link) or you could display the picture like this.

So? Can you live with the reduced voltage out of the 555?

LEDs, 555s, Flashers, and Light Chasers

BTW, where this schematic shows 0.2V is generally incorrect. A transistor turn on in saturation is much less voltage than that, as in well under 0.1V.

#### paw

Joined Jan 11, 2012
18
Hi Bill,

Well yes i mean it doesn't have to output 5 Volts.
I am running simulations in Multisim and I am getting a strange output characteristic:

The LED for the first transistor (left most on diagram) works as normal, as if the circuit shown was powered by supply rails and disconnected completely from the 555.

Now, the LED for the second transistor does not switch on when it should do i.e. at lower voltage inputs to the input...

Im so confused :S

p.s. when measuring the current for my circuit, I get it to be approximately between 10mA and 15mA @ 5V

#### Wendy

Joined Mar 24, 2008
22,932
Ah, I see where your problem is.

It has to do with saturation, and how to achieve it.

A transistor requires 1/10 the current on the BE that you have on the CE. So, if you have 20ma on the CE, then you will need 2ma on the BE.

Ignore the 555 for the moment. It take 1/10 the current on the base to turn a collector emitter on. You will need approximately 4.7KΩ instead of 100KΩ for the base transistors.

#### paw

Joined Jan 11, 2012
18
I actually have 1k resistors on the base of the transistors in my circuit.

As I said the circuit works stand alone with normal power rails :S

The first transistor turns on fine...and the LED turns on fine, but it is the second transistor which does not switch on the second LED on when the first LED turns off

#### paw

Joined Jan 11, 2012
18
Please see the attached circuit for a clearer understanding

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#### Ron H

Joined Apr 14, 2005
7,014
Please see the attached circuit for a clearer understanding

Your schematic doesn't show up. If you are trying to post an image, it has to have a URL. It can't be on your computer. if you want to post an image that exists on your computer, post it by using the Manage Attachments button (must first use the Go Advanced button).
When you post a schematic, it helps if it is an accurate representation of what you have, i.e., if your base resistors are 1k and your supply is 5V, then show those on your schematic. Values actually matter.

paw

#### paw

Joined Jan 11, 2012
18
Hi Ron,

Apologies for the mistake and thanks for the pointer. I think you should be able to see my circuit now attached to my previous post.

Many Thanks,
Paw.

#### Ron H

Joined Apr 14, 2005
7,014
With 1k base resistors, your first LED will be drawing current even when the input voltage is zero. The current flows into the base of the second transistor. You can increase the base resistors to 10k, but you will still get some current.
I ran the sim om LTspice with the base resistors at 10k, and a 555 driving the emitters, and it worked as expected: When the 555 is low, the LEDs flash alternately in response to the input control voltage. When the 555 is high, both LEDs are off.

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