Piezoelectric Braille system with PC parallel port control and HV transistors--- make money online

hey guys been here I thought I'd show

you my latest project this is an

electronic Braille system with two

characters that are controlled by the PC

parallel port so I'll open this up and

show you what's built inside the actual

barrel modules themselves are

commercially made these are made by a

company called meat tech or medtech I'll

put a link in the description the

trouble is the the drivers they come

with are a bit proprietary so I built my

own driver circuit and I'll talk about

that in a minute right now I just want

to show you how these things are built

each pin in the Braille display is

controlled by a piezo electric bender

actuator so what we have here is a long

thin panel or rod almost that flexes up

or down like this when voltage is

applied to it so if I fire this thing up

you can see the rod moving in response

to the voltage applied to it

just like that the trouble is the

voltages have to be pretty high to make

the piezo electric effect work so in

this case the device is rated for about

200 volts a zero volt signal causes the

pin to be up and a 200 volt signal

causes the pin to be down or something

like that NB is the other way so the the

driver circuit has to be a high voltage

amplifier to make this work so here's

some notes I made on the project the

input signal to the amplifier circuit is

just 0 to 5 volts this is coming from a

PC parallel port and I used some 74 HC

series logic chips which all work on a 5

volt positive signal and the output

needs to be 200 volts down to zero and

back up to 200 again so really this is

our input and output signal that we want

from the transistor amplifier and here's

a schematic of the actual amplifier

itself it's really just a transistor

with a resistor going up to a 200 volt

supply and thankfully the the actual

load here the piezo element themselves

are entirely capacitive so when the when

we're supplying 200 volts to it the

current draw is zero after a certain

time and that means we can use a pretty

large value of resistor here because it

doesn't really matter how fast the pin

moves in fact inside the piezo electric

device itself they included a 470 K

resistor just to keep the pins from

snapping I think so charging the piezo

element up doesn't need to happen

quickly and therefore we can use a high

value resistor here and that makes the

circuit a lot easier to design speaking

of design choosing a transistor for this

circuit is not the easiest thing because

you need to handle the high voltage so I

used a ztx 458 which is a relatively low

power but high voltage transistor that I

came across when researching deflection

amplifiers for the scanning electron

microscope project

so it has a nice hefty 400 volt emitter

to collector voltage and the gain is

pretty good to minimum 100 and on the

second page it's you can see that it's

about a typical of about 200 gain which

is great you don't have to give it a lot

of base current and you can get the

thing to turn on quite easily it will

also dissipate a whole watt which we

didn't get anywhere close to in this

project but that's quite nice and this

is about 60 cents so that's that's quite

a nice part to be using I just went

through the power calculations the

resistor only dissipate just under a

tenth of a watt when it's pulled down to

ground and so if there's ground on the

one side of the resistor in two hundred

so that's well in line and then when the

transistor is not conducting total power

dissipation is just about zero the two

chips are 74 HC three seven three chips

which are octal d type flip-flops or

really this sort of latches and so what

I did was I took eight bits from the

parallel port data register and plugged

them into the inputs on both octal D

flip-flops and then took the eight

outputs from each of those chips and

plug those into the basis of the

transistors so I can select between

either character each Braille character

has eight pins and this allows me to

select which character I want to update

just by using the single 8-bit data bus

from the PC parallel port so if we go

back to that program I have I can turn

on the bits on and off in the parallel

port and then select which Braille

character I want to update

I also added these tactile buttons

underneath the Braille characters so

that when these are pushed down the

button depresses ever so slightly and

that can be read off in the parallel

port status register which is input only

for the computer the power supply is a

self-contained unit that's let's see if

we can see the model number here it's an

ultra volt 1c 24 series bunch of other

modifiers on the model number there and

this is adjustable from zero to a

thousand volts and I have it set on

about a hundred and fifty I didn't use

the full 200 volts of the Braille device

I mean it's rated up to 200 but it also

works on less and the voltage is

adjusted with a simple pot here and the

input to this supply is 24 volts DC

which is nice because I can use one of

these an international power adapter

these are actually pretty cool

the plug can be removed and then I can

put like a European power adapter or

plug style in there and it's always 24

volts worldwide so let me know what you

think for future videos let me know if

you're interested more in seeing like

fundamental circuit design stuff or just

a rundown of the projects I've been

building hopefully I'll try to get a

little of each end but feel free to

leave any feedback and I will see you

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