# proximity sensor

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#### audioguru

##### Well-Known Member
Never mind calculating the frequency since it is determined by the output high time plus the output low time that you need to calculate.
The datasheet for a 555 says that the charge time of the timing capacitor is 0.693 times (Ra + Rb) x C, and the discharge time is 0.693 x Rb x C.
Then the discharge time is the shortest so it should be the burst time of 196us or more. You need an RC time of 196 divided by 0.693= 283us. Use a 10nF capacitor and a 30k resistor for Rb for an RC of 300us then the burst timing will be 300us x 0.693= 208us.
Add a 6.2k resistor for Ra then the charge time will be (30k + 6.2k) x 10nf x 0.693= 251us. The total cycle time is 208us + 251uS= 458us and the frequency is 1 divided by 458us= 2183Hz.

But how will you use it to turn the 56k oscillator on and off?

#### varuntejavarma

##### New Member
ok. i shall try these values. I thought of using the circuit posted by Dr. pepper https://www.pcbheaven.com/circuitpages/Long_Range_IR_Beam_Break_Detector/
i am using two 555 timers. i shall set the timer IC1B at 56 khz, the other at 2183 hz.

But how will you use it to turn the 56k oscillator on and off?

As per the circuit, the 56 khz signals would be turned on for 251 us and off for 208 us. right?? I dont know how it functions exactly. Is there anything i should do before starting with the circuit

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#### audioguru

##### Well-Known Member
ok. i shall try these values. I thought of using the circuit posted by Dr. pepper https://www.pcbheaven.com/circuitpages/Long_Range_IR_Beam_Break_Detector/
i am using two 555 timers. i shall set the timer IC1B at 56 khz, the other at 2183 hz.

As per the circuit, the 56 khz signals would be turned on for 251 us and off for 208 us. right??
No.
The circuit and your thinking are backwards.
The burst is when the oscillator is turned on and it should be (208us) shorter than the gap time (251us) when the oscillator is turned off.
If the transistor is changed to an NPN then the timing will be correct:

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• IR transmitter.png
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#### dr pepper

##### Well-Known Member
Sounds good to me, did you try it?

#### varuntejavarma

##### New Member

I have tried the circuit. Using a pot at r1, 30k at r2 and 10nf C, I set the ON OFF period at 450 usec. i.e., 2.222 khz. But , the On period is for 230 us and the OFF period for 220us. How could I reduce the on period.

#### dr pepper

##### Well-Known Member
Try r2 at 27k or maybe less.

#### varuntejavarma

##### New Member
I have tried changing R2 value. But the whole period is changing. How to change only the duty cycle

#### dr pepper

##### Well-Known Member
Yes of course if you change rs you need to change r1 as well to keep the freq the same, amybe try both these components as pots.
Or design a variable pulse width astable circuit using the links I sent a few posts back.

#### audioguru

##### Well-Known Member
Which circuit did you build?
My circuit uses an NPN transistor so that the ON time is shorter than the OFF time then the ON-OFF frequency can be high. But the ON time can be from 10 cycles to 70 cycles of 56kHz. 70 cycles is a long time resulting in a low frequency.

#### varuntejavarma

##### New Member
Which circuit did you build?
.

1. I have built the circuit you posted. the on off time were almost equal. then I have added a 1n4001 diode between discharge and threshold pins and observed a large ON time. so, i adjusted the cap value to 40 nf, r2 to 25 k and varied pot to get a freq of 1.087 khz with on period 210 us and off period 710 us. I did the same with other timer to get 56 khz. But i would like to know if its okay to just adjust the cap , res values with ref to oscilloscope to get the right freq or should i stick to some rules. Am i right od did i miss some basic rules

2. I got a waveform with on period of 210 us filled with 56 khz signals. I got it but now, the led indicating the output is ON irrespective of obstacle. It goes off only when i remove the IR led for the board.

#### audioguru

##### Well-Known Member
but now, the led indicating the output is ON irrespective of obstacle. It goes off only when i remove the IR led for the board.
The TSOP IR receiver output goes low to turn on your LED when it detects IR light. Maybe you have sunlight, a fire or a fluorescent light producing IR light.

#### dr pepper

##### Well-Known Member
Sounds like the infra red is passing round you or reflecting off stuff in the room as the led may now be too bright.
Try increasing R5, also try putting the tssop and/or the led in a tube, (opaque pen casing) to direct the light at the tssop rather than at a wall or around the target.
Yes adjusting the cap value is ok, as long as its over the min for the timer which it is.

#### audioguru

##### Well-Known Member
Yes of course, the IR reflects around the object or person that blocks the IR beam. A TSOP IR receiver IC is very sensitive when it receives bursts of pulses.
My TV remote control receiver IC is probably a TSOP and it works perfectly even if I aim the remote behind me, pretty far away from the TV. The TV is in front of me.
Some people mute a neighbour's noisy stereo by aiming the remote at their house across the street.

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