# using high and low switch together on a load

Thread Starter

#### ahmad1250

Joined Nov 1, 2019
3
hello
in the bellow picture is a test method im trying to make it work
--if succeeded i will implement it in my Led clock project
--im making a four 7 segment led using 12V led strips as 3x3 bits for every segment
--the method to control the the segments is by using the 74595 shift register to call each led
--and using the "high side" "low side" transistors as switch to call the leds
--by making a common amongst the led {common cathode or anode}
any way i have very little information to rely on and need an expert opinion
is the circuit in the diagram correct

Mod: Wrong image posted, Deleted, please repost.E

if yes how can i calculate the resistance value for R1, R2 , R3?
what are the power rating should i be using ?
i used for the low side switch R3=300ohms
and for the high side switch R1=R2=1Kohms
but when i tried it the Q2 transistor was getting really hot
as for the arduino code a simple blink for the digital pin high and low states

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Thread Starter

#### ahmad1250

Joined Nov 1, 2019
3

Mod edit: made image local - JohnInTX

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#### JohnInTX

Joined Jun 26, 2012
4,602
Welcome to AAC!
You need a resistor in the base of Q1. When you turn Q2 on, you get a nearly direct short to ground through the EB diode of Q1. Make the base resistor of Q1 the same value as Q3. Install it between Q1's base and the R1-Q2 collector junction.
R1 can be bigger, try 22K-68K. It is just to swamp out any Q2 leakage current when it's off and pull Q1's base up to +12V to ensure that Q1 is off.
Good luck!

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Thread Starter

#### ahmad1250

Joined Nov 1, 2019
3
thnx alot it worked flawlessly