Hi,
I am pretty new to hobby electronics and am looking for some help:
The task:
To convert a long (approx 90 sec) duration 0-12-0 V square wave into 2 pulses of >5V and >1sec duration - one at onset and one at offset of square wave.
(Why: To drive a 5 or 12 V relay to activate an RF transmitter pulse to switch on a latching RF receiver at start of square wave and then another pulse to switch it off at end of square wave).
The circuit and the oscilloscope traces are got are shown in the attached file.
The problem:
The onset pulse works fine but the offset one is feeble to absent.
The basic differentiator circuit worked fine producing onset/offset spikes at +12V and -10.67V respectively. I then added the bridge to rectify 2 positive signals at +12 / +10 V. I put this into an N -MOSFET BUT as soon as I hook the drain to any load the voltage of the offset spike drops dramatically and always matches the MOSFET gate threshold - therefore not really producing much of an activation.
Can anyone suggest what I am doing wrong here, or a better way of trying to achieve my project.
Many thanks
Saqib

 

there is more than one problem with that circuit. for example it is powered by pulse it is expected to process. when pulse is off, there is nothing mosfet can do to illuminate that LED.

also for transistor to turn on gate has to be positive relative to source terminal.
on rising edge that happens because gate gets positive pulse through upper right diode and source connects to ground through lower left diode in bridge.
on negative pulse, gate is connected to ground through lower right diode while source connects through upper right diode to 137k resistor. if the transistor is turned on, the LED current would be limited by series connection of 220Ohm and 137k.


you could for example use XOR gate and tie input to your pulse, other input to RC delay.

 

Thanks for the reply.
I few obvious errors here I admit.
As I am a novice could I just clarify whether by XOR gate you mean an "exclusive - OR" gate or whether this is another type I should research?

 

yes, XOR gate is "exclusive OR".
basically it allows you to compare two signals and gives high output when the inputs are different. if inputs are same (either both low or both high) output is zero. using simple RC network you can create delayed copy of the signal (and compare it with non-delayed original). of course, it would need to be powered from circuit that is not the monitored signal itself.
you could make the XOR from other gates. for example 4093 is quad NAND gate but with schmidt trigger (just an idea). it is a CMOS device that can work from up to 18V (recommended 3-15V). schmidt trigger is "cleaner" gate. it reshapes the signal with sloppy slopes into digital signal with steep edges.

http://www.ti.com/lit/ds/symlink/cd4093bc.pdf
http://en.wikipedia.org/wiki/NAND_logic

 

Hi Panic Mode
I have looked into your suggestion and I am not sure the XOR gate logic would work. If I understand you correctly I should tie one input to the long duration square wave (A) and the other to my (rectified) RC circuit which produces a positive pulse at onset and offset of square wave (B). Inputs and desired logic table are as shown in the attached file. The first 2 lines are as for a XOR gate but the last 2 are for a XNOR. Actually I just want the output to be high for most of the time that B is high. To use a logic gate approach I am wondering whether to just use a buffer on signal B. Ultimately my goal is to get signal B to drive a relay - it does not have enough juice from the capacitor to do it directly and I can't find a way to drive a transistor/MOSFET without attenuating the signal itself!

 

Thanks Panic mode,
Missed your last reply when composing my last post.
Basically use a basic RC integrator to delay the onset offset of the square wave and put that into the XOR gate.
Yes that would work - thanks

 

Many thanks to Panic mode for the solution above. It is certainly easy to control by use of a variable resistor.
However, for anyone who wants to achieve the same aim but not use logic gates I have tested the attached circuit using nothing but relays and capacitors and it works as well.
When the square wave activates top SPDT relay the capacitor charges through reed relay coil till capacitor is close to fully charged. When square wave ends SPDT relay connects circuit to ground through its NC terminal and capacitor drives the reed relay till it is discharged. reed relay R=1050 Ohm. 220uf gives 0.5sec 2200uf gives 4sec.