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How to detect a frequency

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MikeBrady

New Member
I have a reasonable grasp of basic electronics but am struggling to make progress with a 'simple' frequency detection circuit. I hope someone out there can point me in the right direction.

The basic requirement of this project is to generate an IR light beam with a specific frequency, say 3.5 KHz and be able to detect when this beam is broken.
Generating the IR beam is not a problem - I've got this working ok using an IR diode driven by a 555 timer chip and a transistor. Where I am getting stuck however, is in how to detect the emitted IR light at the specified frequency.
I've found some articles on tuned circuits (using inductor, resistor and capacitor) but I haven't been able to figure out how I could use something like this to
a) detect the presence of the IR light with the 3.5KHz frequency, and
b) enable a high output when the beam is interrupted

I know how to generate the input signal (using a IR phototransistor (and a couple of amplifying transistors) - in fact, I can see the received signal on my oscilloscope, but how to measure the frequency, and detect when it is absent is beyond my level of understanding right now.

Basically, I'm pretty much stumped at the moment and would really appreciate any guidance on what sort of circuit I need to detect this specific frequency
Ideally, there will be an IC out there that will do pretty much what I need. Even better, would be a circuit that somehow(?) uses the same pulses from the 555 chip to drive the receiver, so that the circuit always stays 'in tune'.

Any ideas? Your help is much appreciated.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
If you 'ac' couple an amplifier and configure a low pass/high pass filter that should be OK for an 'interrupted' beam project.
 

JimB

Super Moderator
Most Helpful Member
To detect the 3.5khz, I suggest that you have a look at the NE567. This is a Tone Decoder, which gives an output when the tone is present.
Just looking quickly at the datasheet, the output is an open collector transistor which conducts when the tone is present and turns off when the tone is absent.

JimB
 

suhasm

New Member
a) detect the presence of the IR light with the 3.5KHz frequency, and
You could get rid of all this and use a TSOP1738 instead. It receives Ir modulated at 39khz only. It is very simple to use and very cheap also.

b) enable a high output when the beam is interrupted
You can use "missing pulse detector" circuit to find out when the pulses stop coming. Do a google search for "555 missing pulse detector"
 

hog

New Member
make it simple, go to a tv plunker---they are tuned to 40kc electronic goldmine has both the sender and reciever Lloyd
 

MikeBrady

New Member
Online examples of frequency filters needed

A big thankyou to all for the replies - when I Googled for 'electronics forum' and found this one, it looks like I hit a gold mine!

I've had a quick read about the TSO1738 and NE567 and can understand how they could be used to solve my problem. However, I think I'll have a play around with a more 'basic/hand-on' solution where I'll try to
a) filter the signals detected by the IR receiver to isolate just the desired frequency, then
b) use this as input to a 555 missing pulse detector circuit - the more I learn about the 555, the more I realise what an amazingly flexible component it is.

Apart from finding a couple of calculators for determining the component values for filtering frequencies I haven't been able to find much else on this subject online.

If anyone can point me to some good online reading material that will help me to build the 3.5KHz filter from basic components, then that would be appreciated.

For information, the 2 sites I have found thus far are as follows.

Guitar Pedals: R-C Filter Calculator
Tuned Circuit Calculator
 
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