Deriving 2 separate signals from a mixed signal source


Thread Starter


This is a tough cookie, only for the real nerds.

I have to derive 2 separate signals from a mixed signal source, pref, without timing comps such as R-C because the frequency is not always given.

The signal comprises of 3 stages:
and/or pwm with 20% DC (12V)

I would like to output 2 independent signals,
according to the following pattern;
off (0V) = both outputs off
12VDC(high) = output 1 high
PWM appr.50-100Hz 20% DC = output 2 high
Both outputs must act independently.

I hope someone can give a few good pointers.

Thanks in advance.

Michael Griffin

Is this an automotive related application? If so, what exactly are you trying to accomplish? I may have a suggestion if you can provide more background.
My input is a signal that is low (0V)to start with, then it can go high (12V) for some seconds, which is a state I would like to detect en output, but it can also switch on- and off, between 0 and 12V in a fairly rapid mode, like 50 - 100 cycles. The duty cycle is app. 20%.

Now I would like to create 2 outputs:
1. is activated when the input is 12V
2. is activated when te signal is pulsating

Both outputs are low when no input present.
The 2 signals can be also both present, in that case both outputs are high.
The application is what you probably call curiosity, and to widen my horizon.

Your help will be appreciated.
Best regards, Jack

Marc Sinclair - Germaine Systems


Try a Siemens LOGO! (other small programmable relays are available) Use an ON DELAY timer, with a time constant longer than your pulsing signal to detect your first output. Use a frequency trigger to detect your pulses

Marc Sinclair

Michael Griffin

You didn't answered my questions so I cannot offer much detail. I asked if this was an automotive application and what you are trying to accomplish as your description sounds somewhat similar to some of the automotive control

Since I do not know if this is the case, I can only offer a very general answer. You will either need a small custom electronic circuit, or you will need to measure frequency and voltage level. A filtered voltage sense could detect ON or OFF. A frequency counter would detect the 50 - 100 Hz signal. A voltage with a frequency of zero would indicate the 12V DC level. A frequency in the desired range would indicate the pulsating signal.

I do not understand your description of "both present" on "a signal", so I cannot comment on that.
Hi Marc,

that sounds good, an "on delay" I have already. How to create a freq trigger? is that available in a very small, pref. smd IC? What is LOGO?

Thanks for your help.
Hi Michael,

I don't know what you mean by "automotive".

I agree that a filtered voltage sense could detect ON or OFF. let's say Fo <= 10Hz. Low Pass. I don't need to know the freq. just a high output when it is, let's say, above 10 Hz.

By both signals present, I mean, when the pulsating signal is active, I could be overruled by the much slower +12V signal, which causes the pulsating part to sease. Now the output needs to remember that the pulsating signal is present, but just overruled for a small period of time, so it needs to stay high, normally it will go low, because the pulsating part is not there anymore, not that it is switched off, but simply overruled by the other signalpart.

I hope this makes sense, it drives me mad for sure. Anyway thanks for your efforts.


Michael Griffin

By "automotive", I meant relating to an automobile. Are you modifying an automobile, or testing automobile parts? Some devices in an automobile are controlled by sending a PWM signal (10Hz and 100Hz are common frequencies) from the engine controller. The slave device can signal back a fault by holding the PWM signal down. The engine controller can monitor the PWM signal it is trying to drive, and if the signal is low when the controlled expects it to be high, it knows the slave device is signalling a fault.

The monitoring solutions I am familiar with for the above involve the device which is sending the PWM signal also monitoring the signal and detecting the discrepency. There are several ways of doing this. However, if the PWM signal
originates from the same device that is over-riding the signal, then I don't know how you would really detect this.

A friend of mine found that the standard rear light of his car is also used as brake light. Since the brake light is 21W, and the normal light only 5W, a trick is used to make this work right. So after all this seems to be an automotive case, if that may help. For me this is all new, I never concern myself about automobiles, I just use them to get from point A to point B.

However he noticed that whenever he hooks up a trailer, the trailer lights work the old fashioned way, as one suspect that they use some sensor to establish the fact that either the city lights and/or the brake lights are on, one also starts to wonder how they did the trick.
That's what I call curiosity from my part, as an mediate electronical DIYer I pretend that I have to solve this problem, which is by the way not that easy, as proven.
So the problem is NOT HOW to measure this signal, but HOW to interpret it.

AS said, it is a single signal, with 3 states:
OFF, ON and Pulsating.
Where Off is 0V , On is +12VDC and pulsating is
between 0 and +12VDC with a Freq. of app. 70Hz and a duty cycle of app. 20%
The output needs to control 2 lamps separately, so 2 output signals are needed, derived from this 1 input signal, the outputs are:
output 1 is high when the input signal is +12VDC
output 2 is high when the input is pulsating
both outputs are low when the input signal is off
The pulsating signal will be overruled when the brakes are used, so how can one discriminate this?

I hope this will help to solve this hypothetical case.

Michael Griffin

Making the brake and signal lights work on a trailer is often a problem. This often involves modifying the wiring in the car. There are no real standards for how this works. I would suggest that you look in some auto service
manuals for this one.
this may be overly simple, but for the pulsed signal: could you use a diode and small capacitor to rectify the pulses then detect if a voltage of more than say, 0.6 volts is present with a transistor?

then for the 12v signal: same circuit, but detect voltage over about 9 volts. this will also trigger the pulsed circuit as it is above 0.6 volts. note: this solution requires a 12 volt
source of power for the circuit connected to
the pulsed output as it will not drive the
lamp adequately.
any fool ( me included ) can see that if you
rectify the pulses into a cap, it will store
the peak voltage of the signal ( 12v ).
on reflection, the solution i'm thinking of requires a resistor in series with the diode
and a resistor across the capacitor. this ends
up being an RC circuit with a time constant,
however with careful component choice this
time constant can be suited to a wide
frequency response.

also, a cheap 555 timer circuit can detect
the threshold voltages more reliably than
a transistor circuit.