# can i vary the time constant?

Discussion in 'The Projects Forum' started by onlyvinod56, Mar 1, 2009.

1. ### onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
1
hello,
usually any oscillator or time dependent circuit will have timing components such as RC, RL. What happens if i inject a small voltage into the R or C (in RC)? Does it effect the time period of the system?

For example, consider a 555 timer wired as astable multi. Can i change the frequency by injecting a voltage into the R? Usually resistor is connected between Vcc and capacitor.

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
How are you going to inject a small voltage? Almost by definition that is another resistor.

3. ### onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
1
i'll onnect a 1.5v battery with repect to ground.
see the attachment. There are two options iam showing "a & b".
What will be the output swingings for the two cases? Does the frequency effects to this change?

4. ### onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
1
i forgot to put my attachment. here it is. just check this.

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5. ### wr8y Active Member

Sep 16, 2008
232
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Wire it up, put a scope probe on it and find out!

You would learn more by DOING it, than by talking about it. THEN, you could tell us what it was doing and if anyone wanted to argue, you could tell them, "Well, I am watching it work right now..."

6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Your idea will cause the 555 timer to not oscillate.

In the schematic on the left, the 555 will change states when the charge on C1 reaches 1/3 Vcc and 2/3 Vcc. In your variation on the right, the most voltage that would be seen on C1 is 1.5v, which is less than the minimum threshold (3v).

Your "b" switch position is problematic, because it creates a zero-resistance path from the power source to pin 7, the discharge pin. If your 1.5v source would happen to somehow exceed 6v, pin 7 would try to short it to ground, burning up the IC. Pin 7 is only capable of sustaining a 15mA current.

You can experiment with changing the threshold levels by changing the voltage on pin 5, the "control" pin. Basically, the lower the voltage on pin 5, the higher the output frequency of the timer. By default, pin 5 measures 2/3 Vcc, and is the upper trigger limit.

Internally, there are three 5k resistors connected in series from Vcc to ground. Pin 5 is connected to the junction of the upper two resistors. The lower junction is the lower threshold level.

On a BJT 555, the range for pin 5 is roughly 1.5v to 3/4 Vcc.

7. ### onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
1
right now i dont have an oscilloscope. It will take a week to go to my college due to vacation.

Just iam asking does the frequency effects or not?

8. ### onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
1

Ok, Thats the matter of 1/3 and 2/4 Vcc.
ok leave it.
Iam attaching another circuit here.
I dont want to have a manual control of frequency here.
I want a voltage controlled frequency from this circuit.
As shown in the figure, can i vary the 12v to vary the frequency?

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9. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Have you looked at figures 11 and 12 in the Fairchild datasheet? (Figures 10 and 11 in the National datasheet)

10. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
The schematic is for a 566 function generator IC (sadly no longer available), which is VERY different from a 555 timer.

Here's an example of how you might use a pot on the control pin to change the timer's frequency:

The values are for a Vcc of around 10v, although they'd work for a few volts higher or lower.

R1 provides the adjustment. R2 prevents the control voltage from getting too close to Vcc. R3 prevents the control voltage from getting too close to ground. The 10nF cap keeps the voltage level from fluctuating wildly due to noise.

Alternatively, you could use a single-pole double-throw switch with the common connected to the 555 pin 5, one contact connected to the junction of R1/R2 and the other connected to the junction of R1/R3 for a fast/slow kind of thing.
[eta]
You could also use the output of an opamp or other fairly low-impedance voltage source to control the voltage on pin 5. Something like an LF353 or TL082 wired as a voltage follower (output connected to the inverting input and 555 pin 5, signal/voltage level in to the noninverting input) would work just fine.

• ###### 555-Pin5.PNG
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Last edited: Mar 1, 2009
11. ### onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
1

actually iam not using 555 timer. i thought that 555 & 566 are similar.The schematic i attached in the previous reply is a part of another circuit. Here iam attaching this. U said that 566 is no longer available. Then how can i implement this circuit? i need to control the frequency.

see the page 6 of the datasheet http://www.datasheetcatalog.org/datasheet/philips/HEF4752VP.pdf

12. ### italo New Member

Nov 20, 2005
205
1
if you want to enject a small signal there is pin 2 to do that. somebody schematics of 9v running Vcc and puting 1.5v switch is rediculous it will never do nothing.

13. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Pin 2 is the trigger input. Pin 5 is the pin for frequency modulation.

Raybo, is that you?