Monday, 31 August 2015

Static grass applicator from a bug zapper

Static grass is a method of applying little strands of nylon to a board, to give the appearance of a field or grassy area. The nylon strands are given a static charge before applying, to encourage them to stand up on end, rather than lay flat in a straggly, messy way.

There are plenty of static grass applicators on the 'net and, priced at about £20-£25, they're not exactly going to break the bank. But there's something a bit "suspect" about them.

The idea is that you fill the tea strainer with static grass, connect the other, exposed wire to the board you're applying grass to (usually by driving a small nail or pin into a patch of wet PVA glue) switch on and give it a shake.

The high-voltage potential difference between the (metal) tea strainer and the wet area of (negatively charged) glue creates a static effect, causing the nylon strands (static grass) to stand on end.

It's a relatively simple job, and a relatively simple tool. But there's something about that applicator that looks a bit familiar....

In fact, many people have indeed made their own static grass applicators from a metal sieve and a bug zapper. So we hit the local Aldi and picked up both for less than a fiver!

Opening up the zapper reveals a relatively simple  circuit, containing a transformer, transistor and a few capacitors. One side of the output coils of the transformer are connected to the outer mesh of the bat and the other side connected to an inner mesh. The idea  is that a bug getting caught between the two meshes is instantly "zapped".

Whenever we see a power source, with capacitors and a transistor, already we're thinking "oscillator output". The transformer is a twin coil affair, with a massive step-up effect. The remaining capacitors create a "charge pump" effect, to allow a large voltage to be created from a relatively low voltage source (2 x AA batteries).

In fact, this particular bug zapper creates a pulse of about 800V. From experience, we quickly learned that you don't really want to go touching the two output wires while the device is being powered!

Looking at some of the static grass applicators online, it's obvious that some of them are little more than a "hacked-up" bug zapper - with the zapper meshes removed and a metal sieve connected to one side of the transformer, leaving the other side free to connect to a pin or small nail. It took us less than five minutes to get a similar thing soldered up and re-assembled; giving us a static grass applicator for less than five pounds and a total of about ten minutes work.

So how does it perform?
Here are the results.

It's not bad. But some of the grass looks like it's "wilted over" a little bit. Since the circuit is simply a step-up transformer and a few caps, and given we're pulsing the output, rather that leaving on continuously, we thought we'd push it a little bit and see if it could cope with a higher input voltage (the idea being that we'd get a higher output voltage, and thus increase the static effect). Instead of two AA batteries, giving us 3V, we stuck a PP3, 9V battery on the supply.

The results: