# Converting a variable frequency to TTL HIGH and LOW levels, based on a fixed (possible non-fixed?) frequency

I have a square output from an LM324 op-amp (0 and 5 volts) which has a frequency between 1 to 500 Hz (yes, low frequencies). I would like to find a simple way to have HIGH TTL signal when it is over 50 Hz and a LOW when it is below that. It's not necessary to be so precise, but around 50 Hz will be fine. I know that I can do it by uCs, and I did it. However I have 1500 outputs which I should make a network from, and I would like to make it cheap and fast.

More info about the signal: The frequency is almost fixed at something above 50, say 150Hz or may be 250Hz or so, and the frequency variation is less than 10 Hz, then suddenly it will drop to less than 50, stays for a minute and again back up.

Finally: Is there a way I can control this 50Hz threshold by uC?

• High pass filter? – Linkyyy Mar 16 at 20:14
• nope! it is still a frequency over 50 Hz, I need a near-flat voltage at different levels below and over the treshold. – sina rahbari Mar 16 at 20:21
• on in another word, simply: how to know if a frequency is higher or lower than a value? represent in 0 or 1, 0 or 5 volts? – sina rahbari Mar 16 at 20:24
• A high pass filter sounds like a good start, since it turns any frequency lower than your value to (close to) zero. – Hearth Mar 16 at 20:29
• Consider the 74LS123 retriggerable one-shot. – analogsystemsrf Mar 17 at 1:52

You have multiple alternatives, each one of them with different trade offs.

• The conceptually simplest and most precise one, would be to use a pulse counter.

This is probably what you implemented with your micro controller. Gate a known fast clock with each cycle of the input signal and compare that count to a reference.

• Translate that same idea to the analog domain. Using a ramp as the “counter”.

Start a known slew-rate ramp with an edge of the signal and capture the value on the next edge. Compare this value to a reference. Monolithic latching comparators make this simpler than it sounds.

• Use a retriggerable monostable. With the duration being the comparison value.

A retriggerable monostable with a period of 20ms will remain triggered as long as the signal is above 50Hz, and will generate pulses when it’s below that. A second retriggerable monostable can “filter” these pulses into a continuous signal.

• use a filter slope and a peak detector (just as some cheap FM radios)

A filter slope (high-pass or low-pass will do) attenuates different frequencies differently. By detecting the amplitude of the output signal you can compare to a threshold. The higher the filter slope the higher the frequency gain.

• use a known pulse size to convert to PWM. (Frequency to voltage conversion).

By triggering a known-size pulse with each signal edge (with a monostable or just an edge detector) you convert the signal to PWM whose DC average will be proportional to the frequency. You can then use an analog comparator to detect the transition.

Given that you are talking of thousands of signals, any analog solution might be difficult to use reliably with a desired tolerance. Besides, you will need multiple components for each channel; at least 1 IC and multiple passives.

If you simply program a 14-pin microcontroller to implement just this function on several channels, you can use 1 IC for 5 channels or so. With much better precision, reliability, and repeatability. Even a 6-pin micro at 1 IC per channel would be smaller and simpler.

• Thanks everyone, after trying and calculating the costs!, I find a simple microcontroller the best choice to act as a pulse counter, I used attiny13a, two channels for pulse detectiin and two channels for output. Used watchdog timer and pin change interrupts, everything went well! – sina rahbari Apr 6 at 7:09

One way to solve this problem is to use a re-triggerable one-shot component. You said that absolute tolerance is not super critical and so the typical tolerances of a one-shot may be a suitable and simple means to detect the frequency shift of your signal.

The one-shot needs to be setup with a R/C timing delay just at the timing period of your signal at 50Hz. This would be 20msec. As long as the input trigger signal at the one-shot stays above the 50Hz rate the output of the one-shot will stay re-triggered at a high level. When the input frequency drops below 50Hz the one-shot will start to timeout on each input pulse and its output will start to pulse with each cycle of the lower frequency input.

You could possibly use this as is depending upon what you are using to detect the frequency changes or you could add some additional simple logic to convert the lower frequency pulsing output of the one-shot to a static signal envelope. Below I show an example of using a second re-triggerable one-shot to convert the output pulses of the first one shot to the envelope pulse. In this example the venerable 555 chip is shown being used for the re-triggerable one-shots.

In this simulation the first 555 has its timing components (R3 and C4) set to detect just at 50 Hz. An input below 50Hz causes the output to start pulsing. The input shown here has switched to 25Hz. The second 555 chip has to have its timing components (R1 and C6) set for a delay time a little longer than the period of the lowest frequency expected for the input signal.

If you decide to use this circuit concept it would be a good application of the NE556 dual 555 chip. The NE556 is only marginally more expensive than the NE555 so would be more economical to use in the build out of this circuit.

• Thanks, it looks great from the simulation. most probably i will use this idea. – sina rahbari Mar 17 at 13:54