View Full Version : Sensors

01-02-2008, 08:16 AM
I wanted to know if we can directly connect the TIL 38 and the TIL 81 pair to Vcc instead of giving it to an o/p of Schmitt trigger. A pot is used to change the range if im not mistaken.

01-02-2008, 08:45 AM
@cris1: An (Inverting) Schmitt Trigger when connected to a resistor-capacitor network, acts as a relaxation oscillator. This is used to modulate the IR. The pot you were talking about is used to change the frequency of the resultant square wave by changing the ratio of C and R.

So, yes, you can take out the oscillator circuit, but then your IR will be susceptible to stray, and ambient noise.



01-03-2008, 08:34 AM
Well debu see my circuit is just a TIL38 transmitter forward biased and TIL81 receiver reverse biased. The supply voltage is Vcc. The TIL81 goes to he ground through a pot or a 10 K resistor. The output is taken from in between TIL81 and and the resistor to the comparator. The non inverting i/p of the comparator goes to the ground through the pot. Changing the value of the pot will change the range.

Is this connection right.

01-03-2008, 09:08 AM
@cris1: Seems that way, but do post the schematic, its the only way to be sure. :)

Edit: Now that I think about it. Are you using the sensor pair to measure distance? If so, you would use the potentiometer to adjust the modulation frequency only. Right? The analogue value of the reciever would give you the distance, and the pot, would be used to calibrate. Hmm... Post the schematic, I cant say for sure as yet.



01-03-2008, 11:52 AM
1. It is better to post the schematic. Or it will take days to resolve a very simple problem.

debu! :
Cris is just using the sensor as a simple On-Off switch for line or obstacle detection.

2. Your connections are correct and will work.
3. You have not specified the TIL38 connection to gnd. It will burn without a current drop resistor in series.
4. there is no need for any interface between the sensor and the input circuit.
Usually, the sensors are connected to an interface device ( amplification) for the foll:
a) buffering
b) Impedance matching
c) both of the above and some gain.
d) sensitivity and range, both of which are low in case of diodes on load.

5. The interface is necessary to raise the sensor o/p to TTL levels.
I doesnt matter if it is a Schmidt or ordinary CMOS/TTL.
6. An opamp is used to increase the sensitivity with gain. It is not necessary to give the o/p of the sensors through a TTL interface , if the o/p swing is limited to OR equal to the TTL devise of 5v.

I short, just go ahead and try out your circuit. if it doesnt work, manage the Vcc so that the Vcc=5v + .8v for TTL logic.

...Now, be a good boy and post your Schematic, so that we know if you have the polarities right.

Good Luck!!