A single pulse on power-on?

[Captain Jack]

Hi,

I have a positioner device that drives a satellite antenna, which is controlled by the satellite receiver. When the receiver is switched on, the positioner device wakes up from standby.

What I would like it to do is for the device to stay in standby mode when the receiver is switched on by "pressing" the power button - but electronically. The button is a simple push one which shorts the two connections going to it. The positioner wakes up when 13V to 18V is present on one of the lines.

I would like my circuit to also 'sense' this line, wait a second or so and then "push" the button for around half a second - essentially a delayed pulse? But only do this once - as long as the line has 13 or 18v on it, don't do anything else until the power is taken away and reapplied.

Does this make sense?

Unfortunately, I haven't really done electronics for a loooong time, so need some guidance on creating this circuit. I was thinking of doing it through the usage of logic gates with perhaps a combination of capacitors to introduce the initial second delay and the half second delay for the "pulse/short" but in reality it can probably be something much simpler than this.

Any advice would be appreciated.

Thanks
Alex

 

[KrisBlueNZ]

Hi there and welcome to Electronics Point

This "line" that has 13~18V on it when the positioner starts up... is this a power supply signal? Can this pulse generator circuit be powered from it?

 

[Captain Jack]

Hi there

Yes, I suppose it can be. This 'line' from the satellite receiver actually comes from the LNB port, with voltage being either 18v or 13v. It also carries some commands to control the positioner but we don't need to be concerned about this here. What's important is that the circuit shouldn't interfere with either the satellite signal or the control signals (i.e. not introduce any noise back upstream). Hope this isn't confusing!

So, in short, yes, we could use this line along with some voltage regulator to power the circuit, do a 'pulse' and then become dormant until it's next powered on.

I would probably prefer the circuit to be independently powered, with the line from the satellite receiver being nothing more than a trigger, but I don't really have any logical reasons as to why!

 

[KrisBlueNZ]

OK, sorry about the delay! Here's my suggestion.



The circuit is based around U1, an LM393 dual comparator. This IC contains two comparator circuits, each of which compares the voltages at its inputs and drives its output accordingly - it pulls its output down to 0V if the voltage on its inverting ("-") input is greater than the voltage on its non-inverting ("+") input. In the opposite case, the comparator does not drive its output high, but it allows its output to be pulled high by RP. This is known as an "open collector output".

RA, RB and RC form a voltage divider that provides fixed fractions of the supply voltage to pins 3 and 6 of U1. Pin 6, connected to the junction of RB and RC, sits at around 60% of the applied voltage.

RT and CT form a "time constant" circuit. When power is applied, current flows through RT and charges CT. The voltage at pins 2 and 5 of U1 rises steadily (but not linearly) from 0V towards the applied supply voltage. The right hand comparator in U1 sees its non-inverting input (indicated with a "+" sign) has a lower voltage than its inverting input (the "-" sign). In this situation, the comparator pulls its output (pin 7) low. This pulls Q1's base near 0V and turns Q1 OFF, so no voltage is supplied to the relay coil and the contacts remain open.

After about one second, the voltage at the top of CT reaches 60% of the supply voltage. The right hand comparator in U1 allows its output to be pulled high by RP, and this voltage rises to about 12~13V. It is limited by DL, a 13V zener diode, so that the circuit can be supplied from any voltage from 12V to 30V but will not damage relay K1. Q1 is configured as an emitter follower (common collector) buffer and it pulls its emitter up to about 0.7V below its base voltage. This provides at least 11V to the coil of relay K1, activating it and closing its contacts.

CT continues to charge, and after about another 1/4 second, it reaches the second threshold voltage, set at the junction of RA and RB, at around 68% of the supply voltage. At this time, the left hand comparator in U1 detects that its inverting ("-") input (pin 2) voltage is now higher than its non-inverting ("+") input (pin 3) voltage, and drives its output (pin 1) low. This pulls Q1's base down to 0V again, turning the relay OFF.

The circuit remains in this state until power is removed (at which time CT should discharge through DD) and restored.

C2 (which should have been called CD) is a decoupling capacitor and is required for reliable operation of U1. It must be connected as closely and directly as possible between pins 8 and 4 of the IC. Use a ceramic capacitor.

K1 is a reed relay. Its coil must be rated for 12V and should have a resistance of 1 kΩ. Either of the part numbers given is suitable and both are available from Digi-Key. Relay coils normally have a diode connected across them to absorb the inductive kickback from the coil when it is turned OFF but that is not needed with this circuit arrangement. Some reed relays include an internal diode connected across the coil; this isn't needed, but if it is present, the coil must be connected with the right polarity: positive to Q1 emitter.

 

[Captain Jack]

Kris, this is excellent! Thank you for the detailed description and, of course, the schematic! Let's hope now I don't blow the house up...