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Old 17th Apr 2019, 5:59 pm   #1
Semir_DE's Avatar
Join Date: Jan 2019
Location: Heusenstamm, (Near Frankfurt am Main), Germany.
Posts: 34
Default "PICGEN" pattern generator for 405 - SMD version

Hello fellow 405 enthusiasts,

I will split this post into two parts in order to be able to upload all associated images. This is part 1/2. Hint: the circuit diagram is in part 2/2.

When building the Hedghog converter I stumbled across another one of Franks nice projects, the “PICGEN” pattern generator. I found this to be a very interesting project and went on to build one.

I started with a simple universal board:

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Ever since building the Hedghog I had become a fan of SMD technology, so I decided to make an SMD version of the PICGEN generator I will call it “PICGEN-S”. I not only wanted to make it smaller, but I also changed some parts of the circuit to improve usability. In the following images you can see the (for now) final PCB layout with SMD components.

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Here is a list of changes I have made:

a) Added some low pass filtering to the output signal in order to limit its bandwidth. This will result in less steep rise and fall times of the signal pulses thus reducing the chance of vision buzz. By adding some variable resistors a precise setup of the sync and video amplitudes is possible. Here is an image of the horizontal sync interval as it appears at the output.

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b) Powering from a 5V USB port: By design the output level of this generator depends on the supply voltage. The original design was for a 5V supply. The used PIC processor is, however, specified to run on 3.3V as well, so I changed the design of the circuit to work on 3.3V. This allowed me to put in a 5V to 3.3V voltage regulator (MCP1700) which guarantees a fixed voltage even if the input 5V is slightly off which is often the case with power banks and chargers. I have measured anything from 5.2V down to 4.85V. This image shows the PICGEN and my channel 1 modulator running while being powered from a USB power bank.

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c) More outputs: Instead of the simple single transistor output I added the THS7314 output chip to the design. This is the same output driver that the Hedghog uses. This IC can run on a mere 3V of supply voltage and has 3 output drivers. I used two of them as video outputs and the third as a “Composite Trigger Pulse” (CTP) output. By altering a solder jumper the third output can also be turned into a video output.

To be continued in part 2/2.

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