Infrared proximity detector usign 555 timer

Thread Starter

youngboss

Joined Oct 5, 2012
12

can some one help me understand how the transistor works towards the 9v relay????
the one that has the 0.47uF cap connected
the 555 timer is driving about 36kHz right now
soooo does the module's 2.2kohm but and the 0.47uf has to be set to this frequency like a lowpass filter???
 

wayneh

Joined Sep 9, 2010
17,153
Right, the cap smooths the voltage appearing on the base of the transistor, which in turn controls a larger current to activate the relay. Rule of thumb; the current flowing into the base is 10% of the current through the collector-emitter.
 

Sensacell

Joined Jun 19, 2012
2,890
The RC just provides a bit of smoothing to prevent the relay from chattering.

The IR module does not output 36 KHZ, just a voltage level. In this circuit, the output will be noisy at the threshold of detection, hence the RC on the transistor base.
 

THE_RB

Joined Feb 11, 2008
5,438
It won't work as the output of the IR sensor is a PULL DOWN only. You need to add a pullup resistor, but then it will work backward, ie the relay will be on when there is no IR detected!

Also many sensors will have gain issues if you transmit a continuous signal as your 555 does.

Finally, that sensor is totally wrong for proximity as it will self-adjust gain and is very sensitive. It will just as likely detect a wall at 30 feet or a finger at 1 inch. And it's no good for a break beam sensor as the gain is so high it will detect all the splash IR even when something is directly in the path of the "beam".

Please say what you need the detector to detect, and in what environment. I'm sure people can suggest a better setup.
 

Thread Starter

youngboss

Joined Oct 5, 2012
12
soo please correct me if im wrong guys
the cap is delaying the voltage to charge up so that the transistor would go on non -abruptly right??
sooo when the module aint receiving any signals than there is no voltage coming out of it
that means no current through the transistor making it a open circuit, no current through the relay keeping it off
when there is voltage coming out of the module
cap charges up with i guess 0.7v current going through bias to emitter turning on the transistor, and current runs through the relay switching it on
 

Thread Starter

youngboss

Joined Oct 5, 2012
12
thanks RB
im trying to detect an ogject about a meter away only
like set it up in front of a desk and turns on when someone sits down on their seat
right now im planning to set up the LED and the module pointing the same direction so that the IR beam will bounce off the object towards the module hence turning the module on
 

wayneh

Joined Sep 9, 2010
17,153
sooo when the module aint receiving any signals than there is no voltage coming out of it..
There's an internal 100k pull-up resistor in the detector, so there will be a voltage when no signal is received. it's a high impedance and will be pulled down by your loading. As RB notes, without external pull-up, you'll never see any voltage output to trip the relay. When it sees a signal, it will pull the voltage low, not high.

I don't think the sensor's internal output transistor can sink enough current to trip your relay directly, but the datasheet I found wasn't clear (to me) on that.
 

Thread Starter

youngboss

Joined Oct 5, 2012
12
sooo i should put a resistor to Vcc on the base side of my transistor????

is there any way to predict how long of a distance it would detect?
 

Sensacell

Joined Jun 19, 2012
2,890
Yes... but you could build 5 units in the time it would take to do any meaningful calculation- too many variables.

It will totally depend on the IR reflective qualities of the target, so the range will vary widely.

Placing the emitter and sensor at angles, so the field of view of the sensor and the LED output beam cross at a point in space can make the detection point more predictable.
 
Top