HOBBS SWITCH FIT UP to Rotax 912 ULS2

[Blueadventures]

Hi How can I fitup a hobbs to read engine hours.

 

Is fitting an oil pressure to the engine using a Tridon normally open and closes at 26psi or 20psi ok. If Ok a Tridon TPS031 or 030 could be used to hookup the hobbs ground wire.

 

I see some Rotax 912 setups fit a VDO normally open at the port at the front near where the oil pressure sender is installed; as I have the oil cooler close there I can only fit the above Tridon switches that use the engine body as the earth point.

 

The Skyfox's fitted a VDO brass 'T' and ran the oil pressure sensor and pressure there.

 

Any other working solutions out there in operation?

 

Thanks

 

Mike

 

 

[facthunter]

Don't affect reliability. These oil senders can fail. You can use engine start or leg /airspeed switch. With the latter two you register less hours so might have to make a compensation, as you don't show taxi time. Nev

 

 

[rick morawski]

Hi Mike,

 

One way to do it requires you to make a little circuit board - how's your electronics and soldering skills? Maybe you've got a mate who could make it up for you.

 

It comes as a kit from Altronics. You make up the circuit board, and connect it in parallel with your oil pressure sensor wire, when it senses the rising voltage (settable) it switches on your Hobbs, when voltage drops (engine off) it switches Hobbs off.

 

I haven't used it myself but found it when researching a similar problem, switching an idiot light which you could do as well.

 

Here's a screen grab from the Altronics site

 

 

 

[cscotthendry]

Mike:

 

There's a very easy way to do this.

 

You need a car relay that has "double throw" that means that the centre connector has a contact to connect with when the relay is both enegrgised and de-energised. It's the contact that connects when the relay is de-energised that we are interested in.

 

Connect the centre contact of the switched side of the relay to ground

 

Connect the contact of the de-energised side to the ground contact of your hobbs meter

 

Connect one side of the relay coil to the main switched volts bus.

 

Connect the other side of the relay coil to the "L" connection on the regulator

 

Connect the + side of the hobbs meter to the main switched volts bus.

 

Here's how it works:

 

When the main power is switched on, the "L" output of the regulator goes to ground. With 12 volts on the other side of the relay, the relay activates. Since the connection from the switched side of the relay is to the DEACTIVE side, the hobbs meter won't get grounded and won't run. When the engine starts, the "L" output of the regulator goes to +12V and the relay turns off (the relay has 12v on both sides of the coil) the switch side of the relay then connects on the de-active side and the hobbs meter is grounded through the relay connection and starts running.

 

The "L" output of all automotive regulators is intended for lighting up that little light on the dash when the alternator is not charging, ie when the engine is not running or if the alternator fails. In this case we use it to turn ON the relay and take the ground away from the hobbs meter. When the alternator starts charging, the light would normally go out, but in our case the relay turns off. Because we have a contact on the de-energised side of the relay, it now supplies ground to the hobbs and it starts clocking.

 

I know it sounds complicated, but it's not and it works because I used it in my Nynja. I'll email you a schematic and an explanation later today.

 

 

[Blueadventures]

Thanks Scott

 

I'll do that, look forward to the schematics.

 

I got the prop on yesterday. preping fo paint the flap fairing pieces then atleast prime both sides of each. The rain is staying away the last two days. Then I list and drw aout my electrics and test prior to hooking up.

 

Cheers

 

Mike

 

 

[cscotthendry]

For the benefit of anyone else reading this...

 

 

Here's the diagram of what I described. I have drawn it in its three states.

 

State 1: In this state the main switch is off. There is no volts going to either the relay or the hobbs meter so the meter doesn't run.

 

State 2: In this state, the main switch is on but the engine is not running. In this state, the regulator will ground the "L" output. Since there is 12 volts on the other side of the relay, the relay will switch on and pull the contact down. This stops the ground from connecting to the Hobbs meter. So again the meter doesn't run.

 

State 3: In this state, the main is on and the engine is running. When the alternator starts putting out some voltage, the "L" output of the regulator goes to 12V. Since the relay has 12V on both sides it switches off and the contacts spring up. This allows the ground to reach the Hobbs meter and the meter runs.

 

The relay to do this is readily available at Jaycar or most auto parts stores. The relay has to have at least 5 pins, 2 for the coil and 3 for the switching. It will have 8 pins if it has two lots of switching contacts as shown in the diagram.

 

 

[Blueadventures]

Thanks Scott

 

This will assist me in having accurate engine times.

 

Regards

 

Mike

 

 

[Geoff13]

Can also be done using a couple of resistors, a capacitor and an led as I have done on my X-Air. Will try and dig up the circuit.

 

The LED shows that the regulator is providing output to the battery.

 

 

[cscotthendry]

Can also be done using a couple of resistors, a capacitor and an led as I have done on my X-Air. Will try and dig up the circuit.The LED shows that the regulator is providing output to the battery.

If you've driven the hobbs meter directly off the "L" output of the regulator, that will only work with some regulators. Some regulators supply 12V from that output when the alternator is charging, but some just go open circuit on that output. In the case of an open circuit "L" output, it won't activate the meter.

Edit: I just realised that you're probably using the resistors to supply the 12V which is a simpler solution. Cool!

 

 

[Geoff13]

You got it

 

 

[bizkid]

You got it

Hi. Geoff13.

 

Did you have circuit schematic, like to do the same, sounds like the easy-st way to do it.

 

 

[Blueadventures]

Hi. Geoff13.Did you have circuit schematic, like to do the same, sounds like the easy-st way to do it.

Hi Bizkid

Scotts method on this post also works great; I used his method and works perfect.

 

Also look forward to a copy of Geoffs method.

 

Regards

 

Mike

 

 

[Geoff13]

Hi guys. I am away until Wednesday or Thursday. I will dig out a copy of the circuit when I get home and post it.

 

 

[DGL Fox]

For the benefit of anyone else reading this...[ATTACH=full]36484[/ATTACH]

 

Here's the diagram of what I described. I have drawn it in its three states.

 

State 1: In this state the main switch is off. There is no volts going to either the relay or the hobbs meter so the meter doesn't run.

 

State 2: In this state, the main switch is on but the engine is not running. In this state, the regulator will ground the "L" output. Since there is 12 volts on the other side of the relay, the relay will switch on and pull the contact down. This stops the ground from connecting to the Hobbs meter. So again the meter doesn't run.

 

State 3: In this state, the main is on and the engine is running. When the alternator starts putting out some voltage, the "L" output of the regulator goes to 12V. Since the relay has 12V on both sides it switches off and the contacts spring up. This allows the ground to reach the Hobbs meter and the meter runs.

 

The relay to do this is readily available at Jaycar or most auto parts stores. The relay has to have at least 5 pins, 2 for the coil and 3 for the switching. It will have 8 pins if it has two lots of switching contacts as shown in the diagram.

Hi Scott,

 

This is a great way of doing it in a 19 registered aircraft but what about a 24 registered LSA where you need factory approval for all mods? on my Sportstar the Hobbs is currently hooked up to the master switch which is totally not good, if someone turns on the master and inadvertently leaves it on you can loose many engines hours, it was originally hooked up to the oil pressure switch as described earlier in this thread but it failed and the old owner decided to hook it up to the master and as I said this is not good.

 

Anyone have any alternatives or a more reliable method with a LSA ?

 

David

 

 

[Blueadventures]

Hi Scott,This is a great way of doing it in a 19 registered aircraft but what about a 24 registered LSA where you need factory approval for all mods? on my Sportstar the Hobbs is currently hooked up to the master switch which is totally not good, if someone turns on the master and inadvertently leaves it on you can loose many engines hours, it was originally hooked up to the oil pressure switch as described earlier in this thread but it failed and the old owner decided to hook it up to the master and as I said this is not good.

Anyone have any alternatives or a more reliable method with a LSA ?

 

David

Hi David

If it originlly had a oil pressure switch you could re do that I'd expect; check in the aircraft maunal. The switches are listed on the VDO webb as normally open or closed and what pressure it needs to be activated. The pressure sensors / senders do suffer with vibration causing failures in time. Not a big issue as just replace. Cheap enough (about $30-) to have a spar in the parts box ready.

 

It's suprising that rotax don't have a stock item or method.

 

My old skyfox had the oil pressure switch fitted with a brass "T"; I personally did not like that and searched for a method; then posted this thread and Scott had the perfect solution (IMHO).

 

Cheers

 

Mike

 

 

[Geoff13]

David.

 

I would think that if it was originally fitted with an oil pressure operated switch , that should be shown in the original wiring diagram. If so it could be refitted to original with no other approvals. It would need to be done by a qualified L2 obviously.

 

It may also pay to check if the modified one has approval from the manufacturer or it may not be legal for training or hire. Likewise if that is suspect it may pay to have an L2 go right over it an check for any unapproved variations from original.

 

 

[Geoff13]

This is the circuit that I used. Fairly simple, I didn't make a circuit board or any thing, just wired it all together using wire and heat shrink. Cost about $5 and fits in the wiring loom.

 

The resister value and the electrolytic capacitor where trial and error and I can't remember the final values.

 

The Cap went straight across the meter, the LED is dash mounted and everything else fitted into a heatshrink tube between the two.