Control several binary outputs with raspberry pi, CD74HC273 D-type flip flops and 5v relays

Question

I am using a system with a raspberry pi and multiple d-type flip flop (CD74HC273), its role is to be able to control several digital outputs with a web app.

I use 8 raspbery GPIO bits as data/input bits on each flip flops, and n GPIO bits to select one flip flop and disable the others (with master reset input), so I can control 8 x n digital outputs.

I worked fine when I tested it with leds, so I tried switching to 5v relays, which is the actual goal (below the link to the kind of relay integrated circuit I am using) https://www.amazon.fr/XCSOURCE-Bouclier-Optocoupleur-Arduino-TE213/dp/B00ZR3B252/ref=sr_1_6?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&keywords=relais+5v&qid=1579683668&sr=8-6

But I realized that those are activated by low state input, which is problematic, because 74HC273 output is low state by default if not activated, so activates relays by default, which I don't want.

I am pretty new at electronics, so I don't really know how to fix this problem.

What would be the best solution ? :

• Can I find "active high" relays ?

• Can I find another type of flip-flop that works in the same way, "copying" the input level to the output on clock pulse, but with a "master set" input, all ouputs to high state by default ?

• Should I use 8-bits inverters on every 74HC output ?

• Any great solution I did not think about ? :)

Here is a "sketch" of the circuit (although I don't know exactly what goes on underneath the hood in the integrated relay circuit, that would be the ""?"" bit, but I guess this is a pretty standard component):

EDIT: found the schematics for the "??? relay module":

EDIT : link to supplier documentation for the electrical lock

Thanks a lot !!

Answer 1

I think it would be very helpful for the community if you explain us for which application on the secondary side you want to build this circuit. I'm confused about your "relay" (not a simple relay) shown in your link above. The relay itself is a good isolator, so the primary side (arduino) is completely galvanically isolated from your 12V system and there is no need to add an optocoppler to it. The optocoppler is useless. Rather, the optocoupler needs an additional current, space, costs and last but not least, it also ages and may fail. I also found a good replacement of your shown relay similar but with a high-level trigger e.g. here: https://www.amazon.fr/dp/B07V1X2RSP/ref=psdc_2908498031_t3_B07BVXT1ZK?th=1

But I would build such a circuit by myself. BTW, I hope you'll place some C (10nF or 100nF) to the 74HC273 IC for stabilization the power supply.

^{simulate this circuit – Schematic created using CircuitLab}

Update: @Ouhbelle Probably this is the circuit of the relay I have linked? Because this schematics has two important problems. The transistor's Collector and Emitter have to be swapped. And the diode (LED) in the signal line my cause problems in the long run. You know company Eumig? - They put in something like this to do some sort of rectification. After a couple of months, one after the other devices have failed. And ruined their business. Imagine, if the signal (from 74HC) is 0, and the transistor ages and get higher temperatures, there is a little flow from 5v reverse current from C to the B of the transistor and it could start a current flow heating up the transistor. Remedy: Short-circuit (bypass) the LED and change resistor to a higher value of about 1k - 2k2 or you could place an additional resistor from B to GND.

I think for your application, you do not need a relay - why not using the same circuit which I demonstrated - just exchange the relay with your lock and drive the lock from a 12V at the upper side; then it would be better to use another D1 possibly a 1N4007 etc. ? For higher currents, you could also replace the BC547 with a low-side MOSFET.

Edit: Just a suggestion schematics: https://easyeda.com/oe1tkt/hc273_lock

The R4 is optional but when powering up, it does hold the IRL3103 MOSFET Gate to GND when there are same statics through the contol curcuit. the diode D1 has to be as closed to the LOCK (connector). Additionally a varistor would secure the MOSFET. You should also limit the current through the 12V supply, eventually with a fuce. BTW: Your hand schematics has a little problem with the CLK and MR line (the MR signel is the CLR in my schematics) and with this signal I can reset both or more ICs. With the CLK signal I can choose wich D-FF to set. Compare your and my schematics ....