Someone explain the "dual circuit" hobbs switches.....

[Blazer406]

Your standard garden variety "hobbs" switch is a set of contacts that (in some cases) can be adjusted when the switch "switches". Some are "N.O." or normally open while some are "N.C." or normally closed. Seems simple enough even to my simple mind. The setpoints can be adjustable. This is simple enough and would allow you to control one LED. Without using a relay, I would think you would want the N.O. variety and when it closes... it lights up the LED.

What I want..... is........ 2 small LED lights hooked up so that when I come up to the line..... and start building boost..... the 1st LED lights up at say 4 psi boost..... then the second LED lights up at say 8 psi boost.

In studying the different part number hobbs switches available at "allied electronics", they list some "dual-circuit" adjustable hobbs switches. Basically they have 3 terminals..... have a NO and a NC set of contacts, and they can be adjusted independently of each other. I think this $30 switch might do what I want with only having to purchase one hobbs switch. In this case, I would have to wire a SPDT relay on the NC contact to make the LED light up when that set of contacts broke.

Where it gets "fuzzy" is which part number switch I need.

http://www.alliedelec.com/images/products/datasheets/bm/HOBBS_INC/70111546.pdf

Part no. 77048

Looks like I could set the NO contact to close at 4 psi...... and the NC contact to open at 8 psi...... and be able to tweak either setpoint one way or the other...... In my example.... I'd be targeting a launch boost between 4 psi to just under 8 psi.... which should be obtainable without blowing the tires....

In theory, since I don't have a fancy external gate......I would stage.... begin to build boost.... and when I lit up the 1st LED... I knew I would be at or more than 4 psi..... but not wanting to leave too hard and blow the tires.... I would still want to make sure I wasn't about to leave on too much boost.... which is what the second LED is for. If you blow the tires away at the launch... it is a wasted pass..... so I would want to know if I saw the second LED light up.... that I might be about to launch at more boost than I really wanted to. In my case, my car has HB brakes and it will build 14 psi (track verified and on my datalog) without even trying to push through the lights. Bottom line is.... she won't normally hook on a test-n-tune night on a 14 psi launch..... With a creative LED setup.... it might be easier to find that happy launch boost...... without going over....

IMHO, the LED setup would be easier to launch this than studying a boost gauge. Just something else to try.... Obviously a perfect launch would would be easier with a CO2 powered external gate.... but that is not an option right now.

Anyone played around with a dual circuit hobbs switch?

 

[salvageV6]

The factory issued Hobbs switch for the digital dash would do what you want.

 

[salvageV6]

The Honeywells are dead band of 1 psi. switches you'd have to try one to see if you could get a 4 psi. spread between the contacts you need.

The factory digital dash Hobbs switch might work the best and provide two independant tuneable setpoints although I'm not sure of the repeatability and drift over time on those

Another highly accurate and repeatable method would be to use the MPX4250 pressure transducer and a voltage comparator chip that could light up 2-4 LED's at any set point you desire between 0 and 32 or so psi.

The transducer is about $25 from Newark electronics and the comparator and LED's are about $3.

 

[JimP]

Why not get a two-step and just dial in the boost you want. A bit more money but will always give the same results. Just hold the brakes put your foot to the floor and wait for the green. Just throwing out another option for you to accomplish what your after a good launch.

 

[Blazer406]

Why not get a two-step and just dial in the boost you want. A bit more money but will always give the same results. Just hold the brakes put your foot to the floor and wait for the green. Just throwing out another option for you to accomplish what your after a good launch.

With the internal gate... there is no "launch boost" you can set. In theory... you could put a standard actuator on there and maybe have it as low as 13 or so psi... but it would also limit the upper boost because lack of spring tension even when used with an electronic controller.

While the 2-step would limit the RPM the engine would see.... it won't limit the boost. The boost will still climb even when you are bumping the two-step.

FWIW, I have one of Bob's TR6 ignition systems... just havn't got around to instlling it yet.... and it will 2 step.

I'm wanting to make sure I don't launch at too low of a boost... or too high of a boost..... modulating boost with the gas pedal.....

 

[Blazer406]

The Honeywells are dead band of 1 psi. switches you'd have to try one to see if you could get a 4 psi. spread between the contacts you need.

The factory digital dash Hobbs switch might work the best and provide two independant tuneable setpoints although I'm not sure of the repeatability and drift over time on those

Another highly accurate and repeatable method would be to use the MPX4250 pressure transducer and a voltage comparator chip that could light up 2-4 LED's at any set point you desire between 0 and 32 or so psi.

The transducer is about $25 from Newark electronics and the comparator and LED's are about $3.

I've looked at that.... but I'm a dummy when it comes to integrated circuits.... I hated electronics lab when I was in Engineering School.... and blocked most of that out...... I did learn to play golf though instead

I think I could put together one of these setups.... but I would need the entire circuit drawn out and explained...... FWIW, I studied your other thread in the electronics section and still don't have it straight in my head how I would make it work......

 

[TurboDave]

What I want..... is........ 2 small LED lights hooked up so that when I come up to the line..... and start building boost..... the 1st LED lights up at say 4 psi boost..... then the second LED lights up at say 8 psi boost.

What you have describes perfectly is the hobbs switch on digital dash GN's. And it's adjustable.

 

[Pronto]

Did you look at these Hobbs switches? http://www.racetronix.biz/items.asp?Mc=HOBBS They have a 4, 7 and 15...

 

[salvageV6]

The factory oil pressure switch would probably work with air and trigger at 4 psi. as well.

The factory digital dash Hobbs switch is probably your best bet and the factory points are near what you want out of the box.

I would be happy to calibrate one at those trigger points for ya since I have a setup that works well for adjusting them with shop air well regulated.

TTAowner James may have some used ones lying around cheap enough too he's down the road from where I work a few miles.

 

[DOUGGN]

What you have describes perfectly is the hobbs switch on digital dash GN's. And it's adjustable.

Are those switches still available somewhere?

 

[RmvBfrFlght]

Here you go:



At rest the NO contacts are open and that LED is off. The NC contacts are closed and prevents the other LED from turning on (no current to the LED, and shorting the resister to ground is OK. It is only 12 milli-ampers).

The NO contact closes and that LED turns on. The NC contact opens and the current now goes to that LED turning it on.

Note that the C terminal on the Hobbs switch goes to ground.

RemoveBeforeFlight

 

[TurboDave]

Are those switches still available somewhere?

Don't know if they're still available or not, but here's the part number: 25503871

The switch itself is a grounding-type pressure switch with two terminals. The connector is a brown, two wire, unsealed female style.
Always check the vacuum hose connection to the switch for cracks and leaks when troubleshooting boost display malfunctions. A leak could prevent lights from illuminating.


Pressure turn-on settings are preset from the factory. The pressure settings are adjusted using the set screws shown in this illustration. Adjustment can be
checked using a hand pressure pump with accurate gauge and an ohmmeter. The presets are set to 2.5 psi, and 10 psi,

 

[mgmshar]

I feel a thread hi-jack coming on, but...

I have a digital dash, and I have the above referenced stock hobbs switch. The switch has died and no longer closes when boost is applied, so my lights don't light up any more. I miss the dramatic orange and red lights, especially at night. Scares the helk out of the guy next to me...

I have scoured the internet looking for a new one, but they are no longer available, and nobody can get one.

Does anyone have a known good switch that I could purchase? This switch is the same as what was used in the hot-air cars, so anybody who's parted one of those would also have one.

Has anyone replaced the original GM switch with something else that emulates what the stock switch did?

I agree that the factory GM switch for digital dashes/hot-air cars would be perfect for what the OP wanted. They are just hard to find if you don't already have one!

Thanks,

 

[salvageV6]

Before you toss your original sometimes they can be repaired with contact cleaner and fine tuning the control adjustment and using regulated shop air to control the switch points properly.

Mine didn't work with my digital dash GN I bought used but I was able to get it to work after numerous cleanings and cycling, verified it still works today too.

I'd ask the used parts guys like James or Brian at G-body if they had one.

If they don't have a test setup that would be a hard part to sell used/known good however.

 

[salvageV6]

You could replace the single factory one with two NAPA 0-15psi. ones that you readjusted for whatever two setpoints you wanted and use a T for the hose fittings and make a mount.

701-1577 is the NAPA Balkamp line one, they have three models I believe that are adjustable in various pressure ranges.

 

[Blazer406]

I think I have a factory DD hobbs switch coming.....

Using the theory of the voltage comparitor..... could you use a factory 2 bar map sensor to give you voltages that you could use a comparitor circuit to trigger different things?

 

[RmvBfrFlght]

Sure can. A MAP sensor output is in the 0 - 5 Volt range. Although it typically won't hit the rails. The common LM393 comparator would be what I'd use.

Did you see the circuit in my previous post for use with a Hobbs switch?

RemoveBeforeFlight

 

[Blazer406]

Sure can. A MAP sensor output is in the 0 - 5 Volt range. Although it typically won't hit the rails. The common LM393 comparator would be what I'd use.

Did you see the circuit in my previous post for use with a Hobbs switch?

RemoveBeforeFlight

I plan to try it.... should have the hobbs switch soon.....will try and make something up soon.....

 

[salvageV6]

Yes you can use a typical MAP, the MPX4250 has expanded range and a lot more resolution since it doesn't read vacuum which is useless data in pressure reading situations.

 

[Blazer406]

Wouldn't you know it..... I got my eBoost2 out that I have never installed......and it has... guess what....... 2 programmable outputs to trigger LED lights....... which can be triggered.... by among other things..... boost level.

Go figure.... I had this capability all along and didn't know it.

I hope to install the eBoost2 very soon.

I smell a track trip soon.