UT5A
4
THEORY OF OPERATION
Theory Intro:
The UT5A is made up of many small circuits used to control the 555 timer IC,
which is the heart of the kit. First we’ll cover these circuits individually, then
we’ll place them into the two most basic modes of operation (monostable
and astable) and analyze their operation.
Power Entry:
The UT5A requires an external 5 to 15 VDC DC power supply to operate the
timer circuits. This external DC supply is connected through J1, positive at
pin 1 and negative (GND) at pin 2. J1 pin 3 allows for a separate 3 to 24
VDC supply to be used to set the output signal amplitude parameters or
Output Power. JMP1 allows the Timer Power to be used for Output Power.
Input Circuits:
Triggering the 555 timer IC requires a negative going signal applied to its
input. We have three ways to apply this signal to the IC.
1) NEGATIVE INPUT - If the source signal is negative going it can be
applied to J2 pin3 the boards (-) Input which is attached to C1. For this
option JMP2 (Pull-up Select) may or may not be installed depending on the
source. If installed, it supplies VCC through R1 to C1’s input. J2 pin3 allows
us to bring a negative signal into the timer at C1. With a negative input, D1
is turned off, thus preventing Q1 from effecting the input signal.
2) POSITIVE INPUT - This option requires JMP2 (the Pull-up Select) to be
installed to provide power for Q1. The positive pulse enters through J2 pin 1
(+ Input) current limiting resistor R2 to the base of Q1. The positive signal
turns on Q1. It amplifies and inverts the signal, and now we have a negative
going pulse passed by the steering diode D1 to C1’s input.
INTRODUCTION
The UT5A is most versatile 555 timer kit on the market. The input circuit op-
tions allow for connecting to almost any kind of digital signal. The output driv-
ers allow for connection to digital circuits, relays, indictors, etc... It is designed
to provide the easiest way to apply accurate timing signals to almost any ap-
plication.
The UT5A is also an excellent teaching tool. The detailed theory of operation
will arm you with the knowledge to effectively set up and use the 555 timer.
You will have the chance to experiment with the 555 timer IC, transistor
switches, pull-up resistors, limiter, and differentiator circuits.
In addition, experienced 555 IC users will welcome the practical and truly uni-
versal setup of the PC board.
