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Showing posts from January, 2019

Game Plan Star Trip: Interfacing with the lights

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LDR-1 (Light Driver)

Next up is the LDR as if I lock on one of the lights it won't burn up like a coil. The process of driving the LDR is very similar to driving the SDR so I should be able to take my learnings onto driving the SDR in the future.

The LDR has three 74HC4514 ICs. These ICs have 4 input pins which can configure one of 16 outputs to be active, giving a maximum light count of 48. The Star Trip pinball machine has less than this so some outputs are not connected. The IC also has an enable pin which when set to HIGH will disable all the output pins. The original MCU had the 4 input and 3 enable pins connected via J2. By pushing all three enable pins HIGH the LDR will disable all the lights. The idea of the LDR is you set the 4 pin input to an individual light number, enable the correct 74HC4514 chips enable pin (corresponding to the light you want to light).

If you want to turn on multiple lights at once you actually turn on and off each individual light really fast to …

Game Plan Star Trip: Interfacing with the switches

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Switches In this first detail blog on bringing my Star Trip Pinball back to life I will cover the pinball switches.

The switches are the only item that connected directly to the original MCU-1 Board without a driver board in between. As with most non-modern pinballs the switches are wired as a matrix. You can read more about switch matrixes here.

I recreated part of the original MCU board, in order to interface with the switches, in the same way the old MCU did. While it may have been possible to detect the switches using a different/simpler circuit I knew that if I replicate the circuit from the MCU it should work just fine without changing the existing wiring and switches.

The Circuit
The original circuit was using two LM339 voltage comparators to read the lines and a single 74HC154 to drive the strobes. Both of these ICs are still available at the local electronics store.


The circuit is pretty simple when you boil it down. Lets take a deeper look into the parts.

First 5V power is …