Need Help with Latch Circuit

[Iawia]

Hi All,

I would like to use an RF transmitter to flip a latched circuit.

I found a decent latch circuit using a 555 timer here. I built it and it works great.

My RF receiver provides a momentary 5v signal when the button is pressed. I have attempted to join the wireless signal to the latch circuit using a NPN transistor and a 500 ohm resistor, but it does not work. The LED just stays on once the button is pressed. Can you make a suggestion on how to properly get the wireless signal to latch the circuit? Thank you!

 

[rjenkinsgb]

The voltage difference between C1 and the R1-R2 divider could be either polarity.

A JFET may??? work, with source to the resistors. They can work as a kind of analog switch, under the right conditions.

Other than that, possibly a section of a CD4066 analog switch, or a small relay.

 

[aliarifat794]

For making the remote output latch, you can use a J-K flip flop or an Arduino.

 

[Iawia]

Hi RJ,

I managed find to find a relay (J104A2C 3V) in my drawer. I used a voltage divider to get the 3V for the coil, but I have never seen a relay diagram like this and the pdf isn't too forthcoming. Would this work? Would a resistor be needed at the coil? I'm not an ee and always afraid of burning something up.

 

[alec_t]

Would this work?

No. Those resistor values would require the relay coil to draw zero current to keep 3V across it.
What is the relay coil resistance?
How long does it take for the relay to pull in?
What is the duration of the 5V pulse?
Even with a flyback diode across the coil the receiver might get damaged. A buffer stage between receiver and relay would be advisable.

 

[Reloadron]

For making the remote output latch, you can use a J-K flip flop or an Arduino.

Given a choice I would use a flip flop as mentioned. I would likely use D flip flop like a SN7474 Dual D Flip Flop. Many wireless relay cards have a latching option but yours doesn't. So you click once and a pulse latches high, click again and the output goes low.

Relay boards are pretty inexpensive and a single channel card with a latching mode can be had for about $14 USD.

I would think about just buying a card inline with what you want/need as to features. You can have a single or multiple channel

Ron

 

[eTech]

Hi All,

I would like to use an RF transmitter to flip a latched circuit.

I found a decent latch circuit using a 555 timer here. I built it and it works great.

My RF receiver provides a momentary 5v signal when the button is pressed. I have attempted to join the wireless signal to the latch circuit using a NPN transistor and a 500 ohm resistor, but it does not work. The LED just stays on once the button is pressed. Can you make a suggestion on how to properly get the wireless signal to latch the circuit? Thank you!

The circuit you've shown latches the output high on power up and is reset by the switch.
Is that what you want? I've modified the circuit. See below.

This circuit can be easily modified to be unlatched at power up, and latched by the 5v signal.

 

[crutschow]

I managed find to find a relay (J104A2C 3V) in my drawer. I used a voltage divider to get the 3V for the coil, but I have never seen a relay diagram like this and the pdf isn't too forthcoming. Would this work? Would a resistor be needed at the coil?

Yes, it will work, but you can add a resistor in series with the coil to operate at 5V.
The coil resistance is about 20Ω so add a 13Ω resistor in series for a 5V signal.

 

[alec_t]

Instead of a relay you could use an XNOR gate:-

 

[crutschow]

Instead of a relay you could use an XNOR gate:-

That will work, but if you are adding a CMOS gate, then just go to a CD4013 Flip-Flip configured as a toggle circuit, and eliminate the 555.

 

[Iawia]

The circuit you've shown latches the output high on power up and is reset by the switch.
Is that what you want? I've modified the circuit. See below.

View attachment 146054

This circuit can be easily modified to be unlatched at power up, and latched by the 5v signal.

Etech, yes I would prefer that it is unlatched at power up, what would change? I will attempt this circuit, but what do the circles in front of the RST and TRIG mean? For this solution, it seems I have everything required, and dont need to go out and get more components.

 

[crutschow]

Do you want the circuit to alternately change states with each pulse or just go the the set state and stay there?

 

[eTech]

Etech, yes I would prefer that it is unlatched at power up, what would change? I will attempt this circuit, but what do the circles in front of the RST and TRIG mean?

The circle means that the pin is an active low input pin. It requires a logic low level voltage to activate the function. RST resets the timer when the pin voltage falls below 1v. Likewise, TRIG triggers the timer when the pin voltage drops below 1/3 Vcc.

For this solution, it seems I have everything required, and dont need to go out and get more components.

Here is the circuit with modification to be unlatched at power up. The signal from the rf module will cause the 555 to latch.

The circuit can also be modified so it can be controlled like a flip flop, and latch or unlatch on command.

 

[crutschow]

The circuit can also be modified so it can be controlled like a flip flop, and latch or unlatch on command.

As a toggle flip-flop with just one control input (other than the 1st circuit in post #1)?