# A2 ELEC6 CW - Part 3

##### 2016-12-13 17:15:00 +0000, 1 year and 8 months ago

Back again a week later, here’s another update on what I’ve been up to.

• Redesigned most of the circuit
• Re-built the photodiode array
• Found out something cool about lights
• Wrote some code
• Started on the LaTeX document

## Rethinking the circuit

I spent a few hours over the week to rebuild the circuit in Yenka;

So what’s changed? \

• I changed the phototransitor array from using 311N’s to 741 op-amps.
• Created a logic system so to stop TR1/TR2, or TR3/TR4 being on at the same time (which would destroy the MOSFET’s).
• Generally simplified the system, removing unnecessary components.

## Rebuiling the photodiodes

Basically, the whole array was wrong; to start off I couldn’t get the 311N’s to work as comparators properly, and the circuit I was using was supposed to amplify the output voltage of the photodiode, it didn’t have a very large $V_{gain}$ so I decided I didn’t really need it. Instead I’ve just implemented a simple comparator using 741 op-amps.

## Overhead lights issue

When I got the phototransistors working, I found out that they didn’t switch from high/low immediately. I wired up the output of the comparators to some LED’s and saw that the LED’s light faded on/off, which is very odd since, $V_{out}=A(V_+-V_-)$, therefore any slight difference in voltage is amplified (limited to the supply), so in theory Vout should’ve gone from +Vs instantly.

At first I just thought there was some electrical noise in the circuit, but after thinking and talking about it with some people, we came to the conlusion that the LED’s must be switching on and off very quickly, thus the LED’s brightness looks dim, but not off, this is because the total power to the LED is not enough to power it fully, but enough to keep the persistance of vision, the same way in which multiplexing works.

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