Parachute Deployment Timer

I am busy trying to build a parachute deployment timer for an amateur rocket using 555 ics

What have I done thus far

I have a switch that closes temporarily while the rocket accelerates. I am trying to use a Bistable 555 to latch the acceleration switch and a "delay before on" 555 to drive a MOSFET which will sink current for a piece of nichrome wire.

Basically I am trying to combine the two circuits shown below

bistable latch

bistable 555

delay before on

delay before on 555

I have "breadboarded" both of these circuits individually using LEDs to test the output and they both did what they were supposed to do. When I try combining the two circuits things go skew.

Below is schematic of what I am trying to do with the two 555. my problems begin at Q1 and M1


simulate this circuit – Schematic created using CircuitLab I assume this schematic is fundamentally flawed

So here are my questions

1. How do I use transistor Q1 to ground the reset pin of U2 or is there an easier way to place Q1 so as to stop U2 from starting the delay sequence?

2. How do I drive a power MOSFET with the output U2

  • 1
    \$\begingroup\$ BUK444-60H is an N channel MOSFET, you appear to be using it as a source follower rather than as a switch so at 9V you certainly won't get 9V across the nichrome wire. \$\endgroup\$ Commented May 22, 2016 at 14:56
  • \$\begingroup\$ What JIm says: When you use an NMOS, connect the source to ground and the load to the drain. To turn on apply supply voltage to the gate. For a PMOS: connect source to supply, drain to load and apply 0 V (zero volt = ground) to gate to turn the PMOS on. \$\endgroup\$ Commented May 22, 2016 at 15:29
  • \$\begingroup\$ In addition to the other comments, you could put the nichrome wire in the drain circuit (from drain to power) and ground the source of the FET. \$\endgroup\$ Commented May 22, 2016 at 16:08
  • \$\begingroup\$ My thinking is that the basic logic of the whole operation is wrong. At launch the accelerate switch momentarily operates resetting the first 555 latch (output low, 0V) BUT there is nothing to set the output high when the rocket starts to fall so it remains low. The second 555 isn't triggered through the reset input and as Michael points out Q1 shorts out the 9V supply anyway. Even if it was triggered the output of the second 555 goes high imediately switching on the wire (no delay). What you require is something triggered by the falling edge of the delay. \$\endgroup\$ Commented May 22, 2016 at 18:03
  • \$\begingroup\$ A simple SPDT accelerator switch that charges a capacitor ONLY on acceleration, then dumps its charge to the mosfet when acceleration stops, would make most of these parts unnecessary. \$\endgroup\$
    – user105652
    Commented May 22, 2016 at 21:53

1 Answer 1


The problem with the reset circuit is that you have the reset hard tied to the 9V supply. The Transistor, when it turns in tries to short the 9V to GND.

Remove this connection and place a resistor from 9V to the RESET input. Something like 4.7K to 10K would be appropriate. Now the NPN transistor will be able to operate the reset input.

To make this a complete answer let me also add what others have already said about the MOSFET you are using to drive the nichrome wire. Connect the wire between the 9V supply and the drain of the MOSFET. Then connect the source of the MOSFET to GND. Now you will be able to switch a full 9V across the wire.


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