Ground something, for a short time

I need some simple solution, for grounding an AdKey for a short time lets say 200ms, only once, on each power on.

I don´t know the exact needed time so it would be nice if i can change the time with an resistor or so. Till i found the right time.

I am using that circuit to power on/power off the load.

Is there a way to hold up the grounding from the on/off keypress a bit longer. becaus when i put the AdKey direct behind the on/off button. and u press the right time or make 2 short presses. It triggers. (The mcu take a bit time to bootup.)

Can that solution work?

So after 100ms or so the p-channel JFET(J1) should cuttoff,and stopping the ADKEY Ground. In a couple days my hfet should arrive and i can tell more.

Big Thanks in front.

• What is an AdKey? Jan 16, 2019 at 13:05
• an adkey is something like that astro-video.com/uploads/2/0/4/0/20407741/3501640_orig.jpg?446 , i think it stands for adaptive key. Jan 16, 2019 at 13:11
• What does it mean to "ground an AdKey"? How many pins need to be shorted to ground, what are the high and low voltage levels, and how much current will be passed to ground? You need to tell us what the AD_KEY signal is connected to. Jan 16, 2019 at 15:13
• i need to ground one pin the adkey to ground, when u ground it over 100k ,u get the desired function. its on 3 volt so, 0.03 mA. Jan 16, 2019 at 17:51
• "Can that solution work?" - No. Because the FET will not instantly turn on or off. It will do so slowly in response to the slowly changing gate voltage. Jan 21, 2019 at 14:20

Something I like to do for reset circuits is a one-shot circuit. At least I think thats what it is called. It looks something like this:

simulate this circuit – Schematic created using CircuitLab

V1 would be your supply voltage, and the output of the XOR is your "reset" signal. Basically, when your supply voltage starts to come up, the top input of the xor is high, and the RC circuit is charging up so it will be low. 10 gives a 1 on the output, so this signal is high until the RC circuit charges up. Once that happens, the output of the XOR goes low. The nice thing about this is that you can adjust your on time by fiddling with R1 and C1. Larger RC, larger time on. You can use this to power a fet, although I would suggest a gate driver of some kind as logic gates have super low current output.

• Thanks, That solution will work. but it needs 4 parts. Maybe there is an simpler solution for it? Hope somebody knows one. Jan 16, 2019 at 13:25
• It doesn't get much simpler than an RC time constant... Are you using a microcontroller or have any type of existing control circuitry? Jan 16, 2019 at 14:02
• no i dont have one, my thinking now was using an RC circuit, in compination with an P-Channel Depletion-Mode mosfet . but there are nearly none. Jan 16, 2019 at 14:10
• Can´t i use a single p-channel mosfet . that when in on status shut itself down, via some sort of RC circuit. Jan 16, 2019 at 14:16
• So the circuit you've shown in your original post powers the AdKey directly? As in it is connected to the "5-18V Switched Power" node on the right side? And you want to ground it when you hit the On/Off button? Jan 16, 2019 at 14:18

There are special reset controllers / supervisors available which can be used for such a purpose.

E.g. NPC302 from ON Semiconductor Datasheet.

However, for the NPC302 you need an externally time delay generator.

TI also has such components TI Supervisor - Reset IC category

• Nice, so i need only one part for the job ti.com/lit/ds/symlink/mcp809.pdf . As it puts the reset to low for 240ms, when voltage reaches a threshold. I think thats an real good solution. Somebody knows what happens to the reset pin after the 240ms. Is it dead than? Jan 21, 2019 at 12:09
• @PhaseXavier On page 4 of the datasheet you can see that the RESET pin will sink/source current during reset and source/sink current after that, so it stays active (high or low depending on the chip you select) at all times. Jan 21, 2019 at 12:31
• @JimmyB Thanks dump me so , i think i still need to control a mosfet with the reset pin. Because the Adkey needs to be free to react to other inputs(but maybe the source current(from the reset pin), is low enough to don´t mess with the adkey) At my question, i wrote up another idea with an p channel jfet, that could work. May someone take a look Jan 21, 2019 at 13:12