3-8decoder and relay problem

[Yoshidk]

Hello everyone

im studying electro engineer almost finished first year, and i ran into a problem i dont know how to solve..

This is taken from a larger print, so the actual use is not to power a LED, but it shows the problem

Components:
3-8 decoder non-inverting (74HC238N)
Relay (OMRON G5V-2)

A High signal will trigger a 3-8 decoder to set Y1 to High

Y1 is connected to a relay

I tested the decoder alone and i works as intended
I tested the relay alone and a high signal will trigger it

But together they do not work :S

When its all connected together and i messure the voltage over the relay, its only ~1,5V

But when i disconnect the relay, and messure the voltage on Y1, its 5V

My guess is maybe a voltage division will occur because the internal resistance in the 3-8decoder is larger then the internal resistance in the relay, but if this is the problem, how do i solve this?

Please help if you have any comments or suggestions

Here is the simulation made in multisim
**broken link removed**
(in the picture i use 74HC138N, because my simulation program does not have the 238, but the only difference is the inverting outputs)

-Yoshidk

 

[ericgibbs]

hi and Welcome,
What is the resistance value of the relay coil.??

EDIT:

 

[Yoshidk]

it is 50 ohm

 

[ericgibbs]

it is 50 ohm

hi Yoshidk,
Consider what 'current' the 74HC output pin can supply...... its not going to be 5V/50R = 100mA...

I will not give the answer but you need another device that is a current amplifier between the output and the relay coil,,,, OK.?

 

[Yoshidk]

Before i read your reply, i tried putting an OP-AMP (MCP601) as a voltage follower in between, alone the output in this is 5V, but again when i attach the relay, it doesnt open and the measured voltage drops down

i dont think i have ever heard of a current amplifier..

 

[ericgibbs]

Before i read your reply, i tried putting an OP-AMP (MCP601) as a voltage follower in between, alone the output in this is 5V, but again when i attach the relay, it doesnt open and the measured voltage drops down

i dont think i have ever heard of a current amplifier..

OK.
All you need is a transistor say a 2N2222, connect the base of the transistor to the 74HCT output pin using a series 470R resistor, emitter to 0V and the collector to the relay coil.
The top of the coil to +5V, and IMPORTANT a diode across the relay coil, say a 1N4001.

Look at this clip showing how small the current is from a 74HCT output pin.