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555 timer specialized polarity switcher

123456789a

Mar 2, 2013
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I am inept when it comes to 555 timer circuits however though for my project I will need to use one particularly a circuit that can auto switch from +10 volts and remain so for 5 seconds and then -2 volts and remain so for 2 seconds as it outputs. If someone can give me a schematic with component specs required to achieve said output or atleast a "semi-skeleton" schematic point out to me what part of the schematic will dictate my output and it's time duration and give me the math formula to figure it out I would greatly appreciate it
 

Harald Kapp

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There are plenty of 555 calculators on the intenet, e.g. this one. These typically use a circuit that switches between 0V and Vcc, e.g. 0V and 12V.
This range can be converted to your +10V/-2V range using a variety of methods from simple to complex.
To determine a suitable solution: What are your requirements on output current? At very low current a few resistors can do the job. At higher current an active element like a transistor or opamp may be required.
 

123456789a

Mar 2, 2013
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There are plenty of 555 calculators on the intenet, e.g. this one. These typically use a circuit that switches between 0V and Vcc, e.g. 0V and 12V.
This range can be converted to your +10V/-2V range using a variety of methods from simple to complex.
To determine a suitable solution: What are your requirements on output current? At very low current a few resistors can do the job. At higher current an active element like a transistor or opamp may be required.
My output current requirement would be at minimum 200mA
 

BobK

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It is the maximum that is of interest.

Bob
 

Harald Kapp

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This is one out of many possible ways to achieve your goal:
upload_2016-6-8_7-32-15.png
Parameter values will have to be tuned in detail to your needs.
 

Colin Mitchell

Aug 31, 2014
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In the circuit above, you will find the PNP transistor will now turn OFF and the NPN will not deliver.
 

Harald Kapp

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In the circuit above, you will find the PNP transistor will now turn OFF and the NPN will not deliver.
???
As the simulation shows:
  • For a 555 high output, the PNP is off and the NPN is on as the zener diode becomes conducting.
  • For a 555 low output, the PNP becomes conducting and the NPN id off as the zener diode is operated below its threshold.
 

Colin Mitchell

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You are wrong with your knowledge.

Build the circuit and see it does not work.

I can see it will not work.

We need 1amp , not a few hundred milliamp.

For a 555 high output, the PNP is off ????? Really !!!!!!!
 

Harald Kapp

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For a 555 high output, the PNP is off ????? Really !!!!!!!
Depends on which type of 555 you use. The CMOS types go almost all the way to VDD. Granted, not completely and the value also depends on the output current drawn. An additional zener diode as for the NPN can be inserted to fool-proof the circuit.

We need 1amp , not a few hundred milliamp.
My schematic only shows the principle. As I mentioned below the diagram. 'Parameter values will have to be tuned in detail to your needs.'. I didn't even give concrete types for the transistors. Use darlingtons and you will require less than 1mA base current to drive them into saturation. You will have to increase VDD and lower VSS to account for the higher Vce voltage drop of the darlingtons.
 

Colin Mitchell

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I see you have mentioned a CMOS 555 in your circuit.
The voltage across the 1k for the NPN is ZERO. That will allow a lot of current to flow and the NPN will deliver the 1 amp as expected.
Look up the data sheet for the 555.
 

Harald Kapp

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Is that 4.7K or 47K resistor?
The schematic shows 47k.

You may need to use darlington transistors to get the 1A you need. You may also need to tweak the supply voltages to get the range you need.
As I said, this is a proof of concept which needs to be worked out in the details.
Also, other realizations are possible.
 

123456789a

Mar 2, 2013
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The schematic shows 47k.

You may need to use darlington transistors to get the 1A you need. You may also need to tweak the supply voltages to get the range you need.
As I said, this is a proof of concept which needs to be worked out in the details.
Also, other realizations are possible.
Oh ok, also I am wondering, does the 100uF capacitor have to be non-polarized or can it be polarized? If the latter, which part of its polarity goes to ground?
 

Harald Kapp

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I am wondering, does the 100uF capacitor have to be non-polarized or can it be polarized? If the latter, which part of its polarity goes to ground?
Typically polarized. '-' goes to ground.
 
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