# 555 HYSTERETIC OSCILLATOR help

Discussion in 'The Projects Forum' started by king40, Jul 17, 2013.

1. ### king40 Thread Starter Member

Jul 17, 2013
75
0
555 HYSTERETIC OSCILLATOR

hi

i'm new in forum

i want use this SCHEMATIC DIAGRAM

this is possible to edit this circle to active output frequency by input voltage 0f 0 volt to 15 volt
my means active circle with VCC and GND but active output frequency

by my input (another input) from voltage 0 to 15 volt
this is possible??

2. ### wayneh Expert

Sep 9, 2010
12,374
3,226
Thank you for posting a schematic.

I cannot understand your question. Are you hoping to control the frequency of oscillation by controlling it with a voltage? If so, what frequency range do you want?

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3. ### king40 Thread Starter Member

Jul 17, 2013
75
0
i want convert voltage to frequency with 555
this SCHEMATIC work fine
but just convert VCC voltage to frequency
i want convert voltage separate of VCC voltage
from range 0 volt to 15 volt
output frequency range not important
thanks

4. ### crutschow Expert

Mar 14, 2008
13,475
3,362
You want a VCO (Voltage Controlled Oscillator) such as this.

Couldn't see what circuit you posted in the previous post.

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5. ### king40 Thread Starter Member

Jul 17, 2013
75
0
i want to edit this circuit

this circuit convert VCC voltage to frequency
but i want to edit this circuit to convert another voltage separate of VCC
and from range 0 volt to 15 volt
output frequency range not important

6. ### crutschow Expert

Mar 14, 2008
13,475
3,362
That circuit does not convert Vcc to frequency. In fact, the frequency is relatively independent of Vcc.

You need a circuit similar to what I posted.

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7. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
If you connect your controlling voltage between pin 5 and ground in your first circuit, you'll get a little variation in the output frequency. Depending on how much variation your need over voltage. You'll want to keep the control voltage lower than VCC.

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8. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
If I understand him, he wants to use a 555 with a fixed Vcc of 15V, and to control its frequency with a separate control voltage that goes from 0 to 15V.
This could be done with a voltage-controlled current source which controls the 555 frequency. I think it will need a frequency offset, with a non-zero frequency corresponding to zero volts in.

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9. ### king40 Thread Starter Member

Jul 17, 2013
75
0
Exactly
and when
separate control voltage=0 frequency output=0
frequency output must changed by separate control voltage
this is possible?

10. ### king40 Thread Starter Member

Jul 17, 2013
75
0
i do it
by default output frequency is 3.88khz and when i connect my controlling voltage if my voltage Between 1 volt to 5 volt
output frequency chaneged very low for example show 3.95khz or 3.90khz

11. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
I have designed and simulated several circuits. I'm still fine-tuning them. Be warned that they have a lot of parts (op amps, etc.).

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12. ### king40 Thread Starter Member

Jul 17, 2013
75
0
you can send me your designed cricuits

13. ### Ramussons Active Member

May 3, 2013
561
92
Would something like this work? Use a pair of MOSFET's as a variable resistor.

Ramesh

• ###### VCO.JPG
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14. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
It should change alot more than that. I've fooled around with this and got good variation in output frequency.

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15. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
I think you would need complimentary FET's and differential gate drives to make that work.

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16. ### king40 Thread Starter Member

Jul 17, 2013
75
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this is posible to edit post#5 circuit to work by my point?

17. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Its possible, but just using pin 5 would be easier. Post your failed attempt at using pin5 and I'll tell you what is wrong. I'll even check from my beach motel this weekend if you post.

18. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Here are two VCOs that will output ≈0Hz to 1500Hz as the control voltage varies from 0V to 15V. The one with the MOSFETs requires 3 op amps, so I used a quad, with the 4th op amp used as a voltage follower for the triangle wave.
The one with BJTs only requires 2 op amps, so, if you don't need the triangle wave output, you can use a dual op amp (e.g., LT1366). However, the MOSFET version is a little more linear than the BJT version. Also, you must use matched NPNs, as shown in the schematic.
The simulated frequency vs control data is tabulated on the schematics. This simulation assumes that all resistor values are exact. Tolerance variations will cause (minor?) frequency and duty cycle errors. Capacitor tolerance will affect the frequency. Frequency is inversely proportional to capacitance.
The op amps need to be able to operate on 15V, and have rail-to-rail inputs.

You will need to add 10uF between pins 1 and 8 on the 555, and 100nF between the power pins on the op amp package(s).

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• ###### 555 VCO4.png
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19. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
What you need to do is set the frequency to voltage chart. I'm the one that wrote the Hysteretic Oscillator article. 555s can work as a VCO (Voltage Controlled Oscillator), but not very well. And 0 Hz is not a good option either, you need a real frequency (not DC).

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20. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
It works pretty well with the circuit I posted. Admittedly, it is a complex circuit.

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