One of the nice things of having electronics as a hobby is that the things you make do not necessarily have to be useful. It is all good fun as long as you're entertained. The present project sprang from this mentality. In good Young Ones tradition, the author was very busy scrawling meaningless mottos, maxims and sayings on a flatmate's whiteboard and noticed that his handwriting was barely legible. In addition, the writing was also exceedingly slow. From the idea that it should be possible to complete the activity of vandalising someone else's whiteboard much faster, the idea of the 'electronic stamp' evolved. The function of the felt pen is replaced in the electronic stamp by an inkjet cartridge.
An inkjet printer cartridge these days is an advanced system, with perhaps hundreds of 'nozzles'. A nozzle is a small hole from which a droplet of ink is fired. The method of firing differs from manufacturer to manufacturer. In the case of the Hewlett Packard (HP) cartridge that we use in this circuit, the nozzles are operated thermally.
The print head consists of a slice of silicon, where the nozzles are channels from the ink reservoir to the outside of the print head. In the middle of each tube there is a micro-resistor. By running a relatively high current for a very short time (times of only 10 microseconds are not unusual) through this resistor, the location becomes so hot that the ink that is there evaporates. Because the ink in a gaseous state occupies more space than ink in a liquid state, the (still liquid) ink at the front of the tube is pushed outside. All this happens at a very small scale, as a result if which the speed at which the drop of ink leaves the nozzle is quite high and the ink lands on the paper with high precision.
In a normal inkjet printer the print head is moved back and forth with a motor and a guiding system. If however, we want to make a manageable device then it is not convenient to integrate this entire system into an electronic stamp. That is why we use only the cartridge together with some electronics that drives the head. Moving the head itself is a job for the user. When moving it is important to hold the whole assembly straight and move with a uniform motion across the surface. After a little practice this is quite easy to do.
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