detectiondigital-logicfilteroperational-amplifier
I want to build a simple and ultra low cost circuit that can detect a timed (let's say 200ms) burst of a single sine frequency – say,15KHz. Amplitude will be around AC ~1Vp-p. Detection will give a logic '1' on simple 3.3V TTL output while the single frequency is detected, and '0' when not.
what would be the simplest circuit?
(My guess is a narrow bandpass filter and a logic level comparator/converter, but I'm looking for a simple implementation)
btw, I am running from a single 3.3V supply
It is possible to use a led as a light sensor as well as a light emitter. Essentially a reverse biased led will act as a capacititor, if it is then disconnected the charge will drain at a rate roughly proportional to the light hitting it.
We can use this with a microcontroller - utilising the multi state ability of the ports.
The resistor should be about 100 ohms, I have only used this with red leds - it may work with others.
Use the following sequence:
Set Port A output high
set Port B output low // this makes sure the led is discharged
wait 1mS
set Port A output low
set Port B output high // reverse bias and charge
wait 1mS
set Port B as input // Port B is high impedance input
time how long for Port B to read low
The length of time will be dependant on the amount of light hitting the led.
There are several examples of this on the web - I will list them here as I find them again:
LED senses and displays ambient-light intensity
Red LEDs function as light sensors
Multi-Touch Sensing through LED Matrix Displays - very cool video
Very Low-Cost Sensing and Communication Using Bidirectional LEDs
What you are trying to do is a bit more complicated than you seem to realize. For general unknown I/O pins, it looks like you will need two microcontroller pins. One pin will be a output with maybe 5 kΩ in series. The other pin is a input connected directly to the test chip pin. This allows you to drive the pin high and low, and to leave it undriven. But in all cases you can see what the digital level is with the other microcontroller pin. With this setup, you can detect all the possible digital cases of the pin under test driving high, driving low, or high impedance.
If you stipulate that this test setup is only for digital logic chips and only at the same power voltage as the micrcontroller, then there is no need for analog.
Power and ground pins are another matter, however. These pins need to be tied solidly high or low during operation. In the arbitrary case you will need both low and high side switches on every pin since any of them could be power or ground. This gets messy fast.
I don't want to rain on your idea for a personal project, but frankly this strikes me as quite hard to get right and not much use if you do. 74xxx discrete logic is rarely used nowadays, and when I do need to identify chips I don't find reading the number off the chip all that bad.
How about a capacitor meter? Those little suckers aren't labeled anymore.
Best Answer
Resistors in volume in China cost about 0.1 cent US.
Electrolytic capacitors cost around 1 cent up to around 33 uF.
Some selected ICs cost as little as 2 cents 0 eg TL431, a dual opamp such as LM358 may be 5 cents.
An NE567 would do your job but is a bit rare and may cost 10 - 20 cents.
NJM567 datasheet
I had altogether too much to do with these in another lifetime. They work quite well.
A CD4046 digital PLL would do your job - pricing here and datasheet here - TI and here -Fairchild
Price from Digikey is $US12.8 at 50,000 quantity so Asian price may be 5 cents.
Circuit diagram in real world use is not much more than shown below. Phase comparator two is just an XOR gate but works like magic. The circuit shown below would probably cost under 10 cents in 10,000 + volume in China but PCB cost and manufacturing costs will about double that.
This circuit will work superbly with minimal noise and quite well in modestly noisy circumstances. In very noisy environments a 5th order active filter can be added at a cost of 2 transistors and RC glue - but good quality R's and 's needed for a stable filter - say an additional $0.10.