Drop Watcher Circuit for an Inkjet Test System

Thread Starter


Joined Aug 21, 2008
Hi - I am trying to build a circuit to be used with an inkjet test system but am having problems with getting my circuit to do what I want.

The purpose of the circuit is ultimately for looking at drops ejected from an inkjet printhead at times after the fire pulse signal has been received by the printhead.

I have built my attempted circuit (see attached). Basically what I am trying to do is this:

* Take a psuedo encoder signal from a precision stage controller (pseudo encoder signal is essentially square, freq varying from between 0.5-2kHz, 0.5V amplitude, 100µs duration)
* Then delay that pulse keeping the essential shape, frequency and pulse duration the same. I want to delay the pulse by between 0 & 500µs by using a 555 timer
* Then take the delayed signal and modify the pulse width, also by using a 555 timer.
* This pulse is then used to activate a transistor that will close a circuit containing the strobe LED. (this last part is not shown on my circuit diagram)

I believe my second part of the circuit is correct but I am having problems getting the delay part to work properly. I believe this is because I am using an incorrect operational mode for the 555 timer, but I can''t work out how it should look to work properly.

Can someone please help? I've been at this all week and can't work it out! It is driving me spare :(



Joined Jul 17, 2007
I'm too tired to make a detailed review of your circuit at the moment, but...
Connect pin 4 (reset) of both of your 555's to Vcc.


Joined Apr 20, 2004
The first 555 has 10uF on pin 5 - the recommended value is .01uF. The input is for pulse width control in astable mode.

The second is going to have problems because the output directly drives the LED in the optpocoupler. That current can't safely exceed 50 ma, so a current limiting resistor will be necessary. The output of the optocoupler is pretty weenie, so you may need an extra gain stage or a darlington to be able to control any power.