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555 timer

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andy257

Member
Hi Guys

ive built a 555 timer as shown in the link below. However i have modified it to have a lower mark space ration than 50% by adding a diode in parallel with R2. It works great and i have the correct mark space ration i calculated for this. However when i add the output transistor on pin 3 my whole signal becomes inverted. Without the output transistor the circuit is fine but as soon as i add it the output becomes inverted. Its high for longer than it is lower.

how can i overcome this problem?

there is no led in my circuit just the collector resistor. Using a scope across output to see waveform.

cheers

andy

http://www.technologystudent.com/elec1/5552.htm
 

JimB

Super Moderator
Most Helpful Member
A single stage common emitter amplifier WILL invert the signal.
The output of the 555 goes high for a short duration, putting current into the base of the transistor. This causes the transistor to conduct from collector to emitter, causing a current to flow in the collector resistor.
When there is no collector current, there will be no volt drop across the collector resistor and so the voltage at the collector will be the same as the supply voltage.
When collector current flows, there will be volt drop across the collector resisitor and the voltage at the collector will be reduced, ie, supply voltage - voltdrop in collector resistor.

So what you are seeing is quite normal.
If you want to have a short duration +ve going pulse, an obvious way to fix it would be to just have a short duration -ve going pulse out of the 555.
In other words, the opposite of what you have now.

JimB
 

andy257

Member
JimB said:
A single stage common emitter amplifier WILL invert the signal.
The output of the 555 goes high for a short duration, putting current into the base of the transistor. This causes the transistor to conduct from collector to emitter, causing a current to flow in the collector resistor.
When there is no collector current, there will be no volt drop across the collector resistor and so the voltage at the collector will be the same as the supply voltage.
When collector current flows, there will be volt drop across the collector resisitor and the voltage at the collector will be reduced, ie, supply voltage - voltdrop in collector resistor.

So what you are seeing is quite normal.
If you want to have a short duration +ve going pulse, an obvious way to fix it would be to just have a short duration -ve going pulse out of the 555.
In other words, the opposite of what you have now.

JimB
Thanks for the reply, ive swopped my values around to give me the oposite to what i want. Then the transistor inverts that and gives me back what i require.

cheers

Andy
 

Roff

Well-Known Member
Your transistor may not last too long unless you add a resistor in series with the base to limit the current. Make the base resistor about 10 to 20 times as large (in ohms) as the collector resistor.
How did you get it to work without a connection between pin 7 and the junction of R1 and R2?
 

k7elp60

Active Member
Ron H wrote:How did you get it to work without a connection between pin 7 and the junction of R1 and R2?
As usual Ron is correct.
If it is a normal 555 you do not need the transistor on the output of the 555 as it will drive about a 200 mA load.
 
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