Just Sharing: Monostable Relay

[Lone_Star_Astro(!)]

Last night I finished a super basic 555 monostable multivibrator. I connected it to a relay so I could switch in a power supply instantaneously. The objective being that I want to see how my buck converter responds to that turn-on versus having the voltages ramp up over time like a standard ATX power supply would. I'm pretty proud of it. I know it's elementary compared to a lot of other projects, but I'll be able to use this for years to test other projects. And that's what I like about it. Simplicity with utility.

Some details are left out in this schematic. The transistors forming the darlington pair are 2N2222's. The potentiometer is about 50k and a 100uF cap which allows for up to a 5 second on time for the relay.

Beer cap glued to the momentary push button because I can.

Not soldered in this pic, but these are the guts.

Plugs to connect to the relay.


Let me know what yall think. I'm open to any constructive criticism. Are there ways in which you would do this differently?
Also, I would really like to hear about anybody else's projects that they've built to use for testing purposes or debug. I've heard of some folks creating their own signal injectors to use for debug. I think that's really cool and will probably make one eventually.


Best,
Lone Star

 

[danadak]

Two things :

"Normally" you would put relay in collector leg of Darlington, and
use a series R to limit base current to 1/10 of the collector current.

Also place a suppression diode across relay to minimize transient spikes
due to L of the relay :

Shown here is a single transistor, not a Darlington. If your relay current in the 100 mA or so
just 1 NPN would do.

Diode like a 1N4004 or equivalent.



Regards, Dana.

 

[rjenkinsgb]

I was also going to comment on the lack of a back EMF / flywheel diode.
The relay coil can generate hundreds of volts when suddenly disconnected.

Luckily the darlington setup you used would allow current to circulate after switch-off, unlike a common emitter transistor switch, so it's hopefully not done any damage.