Trying to use RAM... Circuit becomes Light Sensitive???!!

Discussion in 'The Projects Forum' started by dag10, Oct 10, 2009.

  1. dag10

    Thread Starter New Member

    Oct 10, 2009

    I'm planning on building a small Z-80 computer on a few breadboards. Right now I'm just experimenting with RAM to make sure I understand how to use it, as well as bus drivers (or buffers, or whatever they're called).

    I build a simple circuit on a breadboard. It has a write button, an 8-bit DIP switch for the address to write to/read from, a 4-bit DIP switch for the data to write, and 4 LED's to show the output data at the selectedaddress.

    The RAM chip is a 2114N-2L 1Kx4 SRAM chip. Its address pins are connected to the 5v output of the 8-bit DIP switch. It's Data bus from the RAM is then connected to a buffer 74LS244 chip. The buffer's input is connected to the 4-bit DIP data, and it outputs its data onto the data bus. The data bus is then connected to a 4-bit D latch which outputs its memory to the four LED's. The write button is connected to a transistor as a NOT gate. The Output of the NOT gate is connected to the RAM's Write pin (active low), the data buffer's Output Enable Pin (active low), and to the LED's D-latch Latch Enable pin (active high).

    Ok, hopefully you're still with me here. ;)

    The problem is, it sorta doesn't work. I see hints of it working, but it just doesn't work. I can leave the address at 0x00000000, and set the data to 0x0100 for example, and press Write. The LEDs will then immediatly reflect 0x0100. But then, if I lightly tap the wires on the small address bus, the LEDs start lighting up randomly as eigher all on, all off, the data in the data DIP, or some other memory addresses data.

    But wait! That's not all! :eek:

    Here's the super duper odd part: The circuit seems to respond to LIGHT!! If I wave my hand or fingers around the address bus, the LEDs dim, and sometimes change their data as if I was touching the address bus. The same also happens if I tilt the board, or press down on its corners (away from any wires or components).

    I can make a video of it if you'd like.

    Here's more info about my circuit:

    It's based off of Donn Stewart's Z-80 computer at

    Here are the main schematics on that site that I used: (I only used the top RAM)

    Also, I only used 4 bits of data in the data bus. I left the upper four data pins unconnected on the buffer and the LED d-latch.

    I also drew myself a little diagram for myself that shows the 3 ICs and how I connected the pins. I can scan that if you'd like.

    Well, hopefully you're still there. If you're reading this, here's a cookie for you! (Cookie: °). It's a bit small, but I'll give you a bigger cookie if you're able to help solve this mystery. ;)

    Thanks in advanced,
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    Do you have 0.1uF caps across all your ICs power pins? Good power supply filtering?
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    it sounds like you have some floating inputs, or switches without pullup resistors etc.

    Moving your hand near it is changing the electrostatic field and the voltage levels on some input(s).

    Check with a meter and ensure you have a definite 0V and 5V at each input with the switches set in the appropriate positions, and that all input pins have a definite connection to something (eg. ground / 0V if otherwise unused).

    You should never leave any input pins open on CMOS / MOS ICs, as well as strange effects & possibility of static damage it can cause them to draw higher than normal supply currents.
  4. Jack_K

    Senior Member

    May 13, 2009
    Best I recall, The Z80 is not static. I assume its clock is always running?