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The UML 91.305 Development Board provides a collection of useful features for basic digital logic circuit prototyping:

• +5v power supply based on 4 AA cells
• logic probe circuit with red/green indicators for logic high and logic low
• 8 LEDs for monitoring signals
• 8 switches (four DIP, four pushbutton) for generating high and low signals
• serial line voltage converter circuit for interfacing from PC serial ports to chip-level serial signals
• piezo beeper for listening to oscillating signals in the audible range

This document provides an overview of the 91.305 Development Board (UML305DEV). A separate document provides assembly instructions.


This section explains the features of the Development Board in detail. Please refer to the schematic diagram on the last page of this document for additional information.

Power Supply

The power supply is based on 4AA cells, which generate a nominal 6 volts. (This voltage will be somewhat higher when the cells are brand new, and will fall as the cells discharge over use.)

To adjust the voltage to a better range for use with digital logic, the battery voltage is run through diode D1 (a 1N4001 diode). This provides a nominal 0.6v drop, putting the useful battery voltage in the 5.5v range. This is of course higher than the nominal 5.0v specified by digital logic, but within the allowable range.

The 1N4001 diode also acts as a current limit in case of short circuit (power being connected to ground by a wire). In this instance, the diode will get quite hot, and may even fail, but it will prevent dangerously large amounts of current from flowing.

Please DO NOT USE NICAD BATTERIES with the UML 305 Dev Board!! Only use alkaline batteries. Nicad batteries can produce dangerously large amounts of current when shorted.

The Vcc and ground power outputs are available along the right hand connector. The top four positions are Vcc taps; the lower four positions are ground taps. The main power switch SW6 controls power on/off. When power is switched off, both internal board power and external power are removed.

Logic Probe Circuit

A logic probe circuit is located in the lower left corner of the board. The probe signal input is located along the vertical connector strip along the right edge of the board, labeled probe.

The red LED lights when the probe signal is logic high (4.4v or greater).

The green LED lights when the probe signal is logic low (0.6v or less).

If no signal is applied, or if the signal is between 0.6v and 4.4v, neither LED will light.

The circuit is implemented with transistors Q1 – Q4 and the associated resistors.

Q2 and Q3 control the logic high LED. If a signal within 0.6v of Vcc (e.g., 4.4v or greater) is applied, then transistor Q2 turns off. This allows R7 to turn on transistor Q3, and the red LED is lit.

A similar circuit controls the green LED. A signal of 0.6v or less will cause Q1 to turn off. This allows R8 to enable Q4, and the green LED lights.

Eight General Purpose Status LEDs

LEDs 1 through 8 are general purpose status LEDs. Inputs to the LEDs are provided along the right side connector strip. A +5v signal input lights the corresponding LED.

Resistor network RN1 provides 1K current-limiting resistors for each LED.

General Purpose Switches

Switches SW1 through SW4 are four pushbutton switches which generate 0 or 5v signals. Switch SW5 is a 4-position DIP slide switch which generates additional 0 or 5v signals. The switch outputs are located along the right side connector strip.

When a switch is closed (pressed if a pushbutton, or “on” if a DIP slide switch), a 5v output is generated.

When the switch is open, a 0v output is generated.

The 0v outputs are created with 1K resistors to ground (from resistor network RN2). This means that only a limited amount of current can flow through the resistor (5 mA). This is more than enough to ground a digital input, but can’t be used for high current applications.

The 5v outputs are created with the switch closure connecting directly to the Vcc supply. Therefore arbitrary amounts of current can be sourced. The switch outputs are ideal for lighting the status LEDs directly.

Serial Line Converter

The UML305DEV board includes a DB9 female connector suitable for attachment to a PC serial port using a standard DB9F-DB9M straight through cable. The serial signals are converted from the PC voltage standard (–10v for logic high, +10v for logic low) to the chip level (aka “TTL”) voltage standards (+5v for logic high, 0v for logic low). These TTL level signals are available on the right side connector strip at the two points labeled rcv and xmit.

The rcv line is an output of the UML305DEV board and should be connected to the serial receive input of the TTL device (e.g., the 68HC11 microprocessor). It’s the same signal that would be considered the PC’s serial transmit output, only converted from PC to TTL voltage levels.

The xmit line is an input to the UML305DEV board and should be connected to the serial transmit output of the TTL device. It is converted and then guided to the PC’s serial receive input.

Little arrowheads next to the rcv and xmit pins should help clarify which signal is input and which is output to the dev board.

The serial line conversion is done by the Maxim MAX232 serial transceiver chip. The chip uses capacitors to create the +10v and –10v voltages that are supplied to the PC.

Piezo Beeper

The large round piezo beeper will produce clicks and tones when an oscillating signal is applied to its input (labeled piezo on the right side connector strip).

The piezo input is fairly high impedance, meaning very little power is required to generate sound.


The following table lists the components used on the UML305DEV board.

Qty Value Device Part Reference
4 pushbutton sw S1, S2, S3, S4
1 DIP switch S5
1 DB9 jack J1
1 strip header SV1, SV2, SV3, SV4
2 1K resistor pack RN1, RN2
3 1K 1/4W resistor R2, R4, R6
1 2.2K 1/4W resistor R5
2 10K 1/4W resistor R7, R8
2 47K 1/4W resistor R1, R3
2 2N3904 NPN transistor Q1, Q4
2 2N3906 PNP transistor Q2, Q3
1 4AA battery holder BATT1
1 0.1uF capacitor C6
4 10uF electrolytic cap C1, C2, C3, C4
1 47uF electrolytic cap C5
1 1N4001 diode D1
2 HLMP1700 LED3MM (red) LED10, LED12
10 HLMP1790 LED3MM (green) LED1, LED2, LED3, LED4, LED5, LED6, LED7, LED8, LED9, LED11
1 MAX232 serial transceiver IC1
1 DIP1 DIP socket DIP1
1 PIEZOP9924 piezo beeper X1
1 SW101 slide switch SW6


Following are (in order):

• PCB silkscreen (top side printing)
• PCB component side etch
• PCB solder side etch
• circuit schematic

Last modified: Wednesday, 18-Sep-2002 13:24:20 EDT by